Copolymer for polymer electrolyte, and gel polymer electrolyte and lithium secondary battery which include the same

ABSTRACT

The present invention discloses a copolymer for a polymer electrolyte, and a gel polymer electrolyte and a lithium secondary battery which include the same. Specifically, the present invention discloses a copolymer for a polymer electrolyte, which includes a fluorine-based polymer main chain and a unit derived from an acrylate-based monomer or an acrylate-based polymer containing an ion conductive functional group grafted to the fluorine-based polymer main chain, and a gel polymer electrolyte in which lithium ion transfer capability is improved by including the same. Also, the present invention may prepare a lithium secondary battery with enhanced high-temperature safety by including the gel polymer electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No.2019-0096998, filed on Aug. 8, 2019, the disclosure of which isincorporated by reference herein.

TECHNICAL FIELD Technical Field

The present invention relates to a copolymer for a polymer electrolyte,and a gel polymer electrolyte and a lithium secondary battery whichinclude the same.

Background Art

Recently, there is a growing demand for high performance, high stabilitysecondary batteries as electric, electronic, communication, and computerindustries have rapidly developed. Particularly, in line withminiaturization and lightweight trends of electronic and communicationdevices, thin-film and miniaturized lithium secondary batteries, as corecomponents in this field, are required.

Lithium secondary batteries may be divided into a lithium ion batteryusing a liquid electrolyte and a lithium polymer battery using a polymerelectrolyte depending on the electrolyte used.

The lithium ion battery is advantageous in that it has high capacity,but the lithium ion battery is disadvantageous in that, since the liquidelectrolyte containing a lithium salt is used, there is a risk ofleakage and explosion and battery design is complicated to prepare forthe risk.

In contrast, with respect to the lithium polymer battery, since a solidpolymer electrolyte or a gel polymer electrolyte containing a liquidelectrolyte solution is used as the electrolyte, stability is improvedand, simultaneously, flexibility is obtained, and thus, the lithiumpolymer battery may be developed in various forms, for example, in theform of small or thin-film batteries.

A secondary battery, in which the gel polymer electrolyte is used, maybe prepared by the following two methods.

First, after an electrolyte composition is prepared by mixing anoligomer or monomer polymerizable with a polymerization initiator in aliquid electrolyte solution in which an electrolyte salt is dissolved ina non-aqueous organic solvent, the electrolyte composition is injectedinto a battery accommodating an electrode assembly, and gelation(crosslinking) is performed under appropriate temperature and timeconditions to prepare the secondary battery. However, with respect tothe above method, since wetting in a cell is poor due to high viscosityand surface tension problem of the solution before the injection, it isdisadvantageous in that mechanical strength is not easily secured evenafter the gelation.

As another method, after one surface of one of an electrode and aseparator is coated with the electrolyte composition and curing(gelation) is performed by using heat or ultraviolet (UV) light to forma gel polymer electrolyte, an electrode assembly is prepared by windingor stacking the electrode and/or the separator on which the gel polymerelectrolyte is formed, the electrode assembly is inserted into a batterycase, and the secondary battery may then be prepared by re-injecting aconventional liquid electrolyte solution thereinto.

However, this method requires a heat or UV irradiation process forgelation and has a limitation in that the gel-coated separator absorbsmoisture to degrade performance and stability of the battery.Furthermore, since a polyethylene separator, which has been used as aconventional separator, has a high thermal shrinkage rate, a shortcircuit occurs between the positive electrode and the negative electrodewhen the temperature rises under abnormal conditions of use, and thus,the stability of the battery may be reduced.

Therefore, there is a need to develop a method which may securemechanical strength and ion transfer capability and may simultaneouslyprepare a gel polymer electrolyte having improved safety againstexternal impact.

PRIOR ART DOCUMENT

Korean Patent Application Laid-open Publication No. 2007-0051706

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a copolymer for a polymerelectrolyte which includes a fluorine-based polymer main chain and aunit derived from an acrylate-based monomer or an acrylate-based polymercontaining an ion conductive functional group grafted to thefluorine-based polymer main chain.

Another aspect of the present invention provides a composition for apolymer electrolyte which includes the copolymer for a polymerelectrolyte.

Another aspect of the present invention provides a gel polymerelectrolyte which is prepared by polymerization of the composition for apolymer electrolyte.

Another aspect of the present invention provides a lithium secondarybattery in which high-temperature stability is improved by including thegel polymer electrolyte.

Technical Solution

According to an aspect of the present invention, there is provided acopolymer for a polymer electrolyte which includes a compoundrepresented by Formula 1:

In Formula 1,

R_(a1) to R_(f1) are each independently hydrogen, a fluorine element, oran alkyl group having 1 to 10 carbon atoms,

R₁ is hydrogen or a substituted or unsubstituted alkyl group having 1 to4 carbon atoms,

R₂ is hydrogen or an alkyl group having 1 to 5 carbon atoms,

Y is an alkyl group having 1 to 15 carbon atoms which is substituted orunsubstituted with at least one halogen element, an alkenyl group having2 to 15 carbon atoms which is substituted or unsubstituted with at leastone halogen element, a cyclic alkyl group having 3 to 10 carbon atoms, aheterocyclic group having 3 to 10 carbon atoms, a cyclic ether grouphaving 3 to 10 carbon atoms, a heterocyclic ether group having 3 to 10carbon atoms, —(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH (R₁₈ and R₁₉ each are hydrogenor an alkyl group having 1 to 5 carbon atoms, and d and e are eachindependently an integer of 0 to 10, but are not 0 at the same time),—(CR₇R₈)_(g)—O—(CR₉R₁₀)_(h)—CH₃ (R₇ to R₁₀ each are a substituted orunsubstituted alkylene group having 1 to 15 carbon atoms, and g and heach are an integer of 1 to 10), an aryl group having 6 to 12 carbonatoms, —(CH₂)_(f)—CN (f is an integer of 0 to 10), —(CH₂)_(i)—O—CH₂═CH₂(i is an integer of 0 to 10), —(CH₂)_(j)—Si(R₂₀)_(k)(OCH₂CH₃)_(3-k) (R₂₀is hydrogen (H), j is an integer of 1 to 10, and k is an integer of 1 or2), —(CH₂)_(w)—NCO (w is an integer of 1 to 10), —CH₂CH₂—N(CH₃)₂,—(CH₂)_(x)-A (A=—OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 10) orC(═O)(CH₂)_(z)COOH (z is an integer of 1 to 10)),

m is an integer of 1 to 2,300, and

o1, p1, q1′, q2′, and n are the numbers of repeating units,

wherein n is an integer of 1 to 2,000,

o1 is an integer of 0 to 400,

p1 is an integer of 0 to 400,

q1′ is an integer of 1 to 300,

q2′ is an integer of 0 to 300, and

o1 and q2′ are not 0 at the same time.

According to another aspect of the present invention, there is provideda composition for a polymer electrolyte which includes the copolymer fora polymer electrolyte, a non-aqueous electrolyte solution, and apolymerization initiator.

According to another aspect of the present invention, there is provideda gel polymer electrolyte for a secondary battery, which is prepared bythermal polymerization of the composition for a polymer electrolyte, anda lithium secondary battery including the same.

Advantageous Effects

According to the present invention, a copolymer for a polymerelectrolyte, which does not cause internal resistance of an electrodeand has improved solubility in an electrolyte solution, may be providedby grafting an acrylate-based monomer or polymer containing an ionconductive functional group to a fluorine-based polymer, a main chain.Also, since the copolymer for a polymer electrolyte is included, acomposition for a polymer electrolyte, which may form a stable solidelectrolyte interface (SEI) on a surface of the electrode by not onlyfacilitating radical scavenge by fluorine atoms but also by improvinglithium ion transfer capability and wetting to the electrode, and a gelpolymer electrolyte prepared therefrom may be provided. Furthermore, alithium secondary battery with improved high-temperature stability maybe prepared by including the gel polymer electrolyte.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

The technical terms used in the present invention are used only forexplaining a specific exemplary embodiment while not limiting thepresent invention. The terms of a singular form may include plural formsunless referred to the contrary. In the present invention, it will befurther understood that the terms “include,” “comprise,” or “have”specify the presence of stated features, numbers, steps, processes,elements, components, or combinations thereof, but do not preclude thepresence or addition of one or more other features, numbers, steps,processes, elements, components, or combinations thereof.

In the present specification, the expression “unit” or “repeating unit”denotes a basic monomer unit constituting a polymer.

Unless otherwise specified in the present invention, the expression “*”denotes the same or different atom or a portion connected between endsof a formula.

Also, a halogen element in the present specification includes a fluorineelement.

A fluorine-based polymer used in a conventional polymer electrolyte isadvantageous in that it has electrochemical stability even at a highvoltage (5.0 V) when used in a lithium secondary battery and radicalscavenge by a fluorine group is possible. Also, in a case in which thefluorine-based polymer is included as an additive in a liquidelectrolyte solution, improvements in high-temperature and high-voltagestabilities of the secondary battery by the fluorine-based polymer maybe expected.

However, since the fluorine-based polymer has low surface energy, it isnot only difficult to prepare the fluorine-based polymer as aninjection-type electrolyte solution, but penetration of thefluorine-based polymer into an electrode is also not easy even if thefluorine-based polymer is injected, and thus, the fluorine-based polymeris disadvantageous in that it is difficult to achieve uniform stabilitybecause uniform distribution of the fluorine-based polymer on a surfaceof an electrode active material is difficult.

In order to overcome the disadvantage of the fluorine-based polymer, thepresent invention aims at providing a branched copolymer for a polymerelectrolyte with improved solubility in the electrolyte solution bygrafting an acrylate-based monomer or polymer containing an ionconductive functional group to a main chain of the fluorine-basedpolymer. Also, the present invention aims at providing a composition fora polymer electrolyte with excellent electrode wetting by using thesame.

That is, since the copolymer for a polymer electrolyte of the presentinvention has a smaller radius of gyration of a chain and more improvedsolubility in the electrolyte solution than a linear fluorine-basedpolymer having the same molecular weight, which contains thefluorine-based polymer as a main chain, due to a branched structure, itmay be used in the form of a composition for an injection-type polymerelectrolyte, and, as a result, it may be applied and distributed in auniform form on a surface of the electrode.

Thus, if the copolymer for a polymer electrolyte of the presentinvention is used, mechanical strength and lithium ion transfercapability are improved, and a gel polymer electrolyte capable offorming a stable solid electrolyte interface (SEI) on the surface of theelectrode may be prepared. Furthermore, a lithium secondary batteryhaving improved high-temperature stability and safety against externalimpact may be prepared by including the same.

Copolymer for Polymer Electrolyte

First, in the present specification, provided is a copolymer for apolymer electrolyte which includes a fluorine-based polymer main chainand a unit derived from an acrylate-based monomer or an acrylate-basedpolymer containing an ion conductive functional group grafted to thefluorine-based polymer main chain.

The copolymer for a polymer electrolyte of the present invention mayinclude a fluorine-based polymer represented by the following Formula 3as a main chain structure.

In Formula 3,

Ra to Rf are each independently hydrogen, a fluorine element, or analkyl group having 1 to 10 carbon atoms, and

o, p, q1, and q2 are the numbers of repeating units,

wherein o is an integer of 0 to 400,

p is an integer of 0 to 400,

q1 is an integer of 1 to 300, and

q2 is an integer of 0 to 300.

The fluorine-based polymer is a polymer in which a branched chain mayform a grafting structure by atom transfer radical polymerization,wherein, as a representative example, the fluorine-based polymer mayinclude a polychlorotrifluoroethylene (PCTFE) unit as an essential unit.Also, in addition to the polychlorotrifluoroethylene unit, thefluorine-based polymer may further include at least one unit selectedfrom the group consisting of polyvinylidene fluoride (PVDF), polyvinylfluoride (PVF), polytetrafluoroethylene (PTFE), polytrifluoroethylene(PTrFE), and poly-1,2-difluoroethylene.

Specifically, polychlorotrifluoroethylene (PCTFE), poly(vinylidenefluoride-co-chlorotrifluoroethylene (P(VDF-co-CTFE)), or poly(vinylidenefluoride-co-chlorotrifluoroethylene-co-trifluoroethylene)(P(VDF-co-CTFE-co-TrFE)) may be used as the fluorine-based polymer.

That is, in the present invention, fluidity of the polymer chain may beimproved by reducing crystallinity of the fluorine-based polymer bygrafting a functional group cable of conducting lithium ions, such as anacrylate-based monomer, as a side chain onto the fluorine-based polymerthrough atomic transfer radical polymerization (ATRP). As a result, acopolymer for a polymer electrolyte with high solubility in theelectrolyte solution may be prepared. Thus, if the copolymer for apolymer electrolyte with high solubility in the electrolyte solution isused, since a state, in which the copolymer is uniformly dissolved inthe electrolyte, may be maintained while the copolymer does not cause anincrease in resistance in the electrode, the copolymer for a polymerelectrolyte may be uniformly distributed on the surface of theelectrode. Thus, a gel polymer electrolyte, which may form a stable SEIand may simultaneously improve a radical-scavenging action by a fluorine(F) group and high-temperature stability of a positive electrode, may beprepared.

Also, the copolymer for a polymer electrolyte of the present inventionmay include a unit derived from an acrylate-based monomer or anacrylate-based polymer containing an ion conductive functional group asa unit grafted to the fluorine-based polymer as a main chain.

Specifically, the acrylate-based monomer or the acrylate-based polymeris an acrylate-based monomer in which a poly(alkylene oxide) group isadded as an ion conductive functional group in its structure, wherein itis characterized in that it has high surface energy. Thus, if the unitderived from the acrylate-based monomer or the acrylate-based polymercontaining the ion conductive functional group is grafted on the linearfluorine-based polymer as the main chain, surface energy of theconventional linear fluorine-based polymer is improved so that acopolymer for a polymer electrolyte with improved solubility in theelectrolyte solution may be prepared. In this case, the solubility ofthe copolymer for a polymer electrolyte in the electrolyte solution maybe appropriately controlled by controlling the structure of the graftedunit derived from the acrylate-based monomer or the acrylate-basedpolymer.

The acrylate-based polymer means a copolymer which includes a repeatingunit derived from the acrylate-based monomer containing a poly(alkyleneoxide) group in an amount of 5 wt % or more based on a total weight ofthe acrylate-based polymer, wherein the acrylate-based polymer mayinclude the repeating unit in an amount of 5 wt % to 90 wt %,particularly 10 wt % to 50 wt %, and more particularly 15 wt % to 50 wt%.

In this case, the expression “repeating unit” denotes a state in whichthe corresponding acrylate-based monomer undergoes a polymerizationreaction to form a skeleton such as the main chain or side chain of thecopolymer.

Specifically, the copolymer for a polymer electrolyte of the presentinvention may be represented by Formula 1 below.

In Formula 1,

R_(a1) to R_(f1) are each independently hydrogen, a fluorine element, oran alkyl group having 1 to 10 carbon atoms,

R₁ is hydrogen or a substituted or unsubstituted alkyl group having 1 to4 carbon atoms,

R₂ is hydrogen or an alkyl group having 1 to 5 carbon atoms,

Y is an alkyl group having 1 to 15 carbon atoms which is substituted orunsubstituted with at least one halogen element, an alkenyl group having2 to 15 carbon atoms which is substituted or unsubstituted with at leastone halogen element, a cyclic alkyl group having 3 to 10 carbon atoms, aheterocyclic group having 3 to 10 carbon atoms, a cyclic ether grouphaving 3 to 10 carbon atoms, a heterocyclic ether group having 3 to 10carbon atoms, —(CH₂)_(d)—(CR₁₈R₁₉)_(c)—OH (R₁₈ and R₁₉ each are hydrogenor an alkyl group having 1 to 5 carbon atoms, and d and e are eachindependently an integer of 0 to 10), —(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ andR₈ each are a substituted or unsubstituted alkylene group having 1 to 15carbon atoms, and g and h each are an integer of 1 to 10), an aryl grouphaving 6 to 12 carbon atoms, —(CH₂)_(f)—CN (f is an integer of 0 to 10),—(CH₂)_(i)—O—CH₂═CH₂ (i is an integer of 0 to 10),—(CH₂)_(j)—Si(R₉)_(k)(OCH₂CH₃)_(3-k) (R₉ is hydrogen (H), j is aninteger of 1 to 10, and k is an integer of 1 to 3), —(CH₂)_(w)—NCO (w isan integer of 1 to 10), —CH₂CH₂—N(CH₃)₂, —(CH₂)_(x)-A(A=—OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 10) orC(═O)(CH₂)_(z)COOH (z is an integer of 1 to 10)),

m is an integer of 1 to 2,300, and

o1, p1, q1′, q2′, and n are the numbers of repeating units,

wherein n is an integer of 1 to 2,000,

o1 is an integer of 0 to 400,

p1 is an integer of 0 to 400,

q1′ is an integer of 1 to 300,

q2′ is an integer of 0 to 300, and

o1 and q2′ are not 0 at the same time.

In this case, the halogen element may be a fluorine element.

Specifically, in Formula 1, Y is an alkyl group having 1 to 10 carbonatoms which is substituted with at least one halogen element, an alkylgroup having 3 to 10 carbon atoms which is unsubstituted with an halogenelement, an alkenyl group having 2 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element,—(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH (R₁₈ and R₁₉ each are hydrogen or an alkylgroup having 1 to 5 carbon atoms, and d and e are each independently aninteger of 0 to 10), —(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ and R₈ each are asubstituted or unsubstituted alkylene group having 1 to 15 carbon atoms,and g and h each are an integer of 1 to 10), —(CH₂)_(i)—O—CH₂═CH₂ (i isan integer of 0 to 10), or —(CH₂)_(w)—NCO (w is an integer of 1 to 10).

In this case, the halogen element may be a fluorine element.

In Formula 1, a weight ratio of the repeating unit q1′: the repeatingunit o1: the repeating unit p1: the repeating unit q2′ may be in a rangeof 1:0:0.01:0.01 to 1:100:100:300.

In this case, when the weight ratio of each of the repeating unit o1 andthe repeating unit p1 to the repeating unit q1′ is greater than 100, orthe weight ratio of the repeating unit q2′ to the repeating unit q1′ isgreater than 300, since an effect of reducing the crystallinity of thefluorine-based polymer electrolyte is insignificant, the fluidity of thepolymer chain is reduced, and thus, solubility in the electrolyte may bereduced. Also, if the weight ratio of each of the repeating unit p1 andthe repeating unit q2′ to the repeating unit q1′ is less than 0.01,since a radical-scavenging effect by the fluorine group isinsignificant, high-temperature, high-voltage stability of the batterymay be reduced.

More specifically, the copolymer for a polymer electrolyte of thepresent invention may be represented by Formula 2 below.

In Formula 2,

R_(a1)′ to R_(f1)′ are each independently hydrogen or a fluorineelement,

R₁′ is hydrogen or a substituted or unsubstituted alkyl group having 1to 3 carbon atoms,

R_(4a) to R_(4c) are each independently hydrogen or a substituted orunsubstituted alkyl group having 1 to 3 carbon atoms,

R₂′ is hydrogen or an alkyl group having 1 to 5 carbon atoms,

Y′ and R_(5a) to R_(5c) each are an alkyl group having 1 to 15 carbonatoms which is substituted or unsubstituted with at least one halogenelement, an alkenyl group having 2 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element, a cyclicalkyl group having 3 to 10 carbon atoms, a heterocyclic group having 3to 10 carbon atoms, a cyclic ether group having 3 to 10 carbon atoms, aheterocyclic ether group having 3 to 10 carbon atoms,—(CH₂)_(d)—(CR₁₈R₁₉)_(c)—OH (R₁₈ and R₁₉ each are hydrogen or an alkylgroup having 1 to 5 carbon atoms, and d and e are each independently aninteger of 0 to 10), —(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ and R₈ each are asubstituted or unsubstituted alkylene group having 1 to 15 carbon atoms,and g and h each are an integer of 1 to 10), an aryl group having 6 to12 carbon atoms, —(CH₂)_(f)—CN (f is an integer of 0 to 10),—(CH₂)_(i)—O—CH₂═CH₂ (i is an integer of 0 to 10),—(CH₂)_(j)—Si(R₉)_(k)(OCH₂CH₃)_(3-k) (R₉ is H, j is an integer of 1 to10, and k is an integer of 1 to 3), —(CH₂)_(w)—NCO (w is an integer of 1to 10), —CH₂CH₂—N(CH₃)₂, —(CH₂)_(x)-A (A=—OC(═O)(CH₂)_(y)COOH (y is aninteger of 1 to 10) or C(═O)(CH₂)_(z)COOH (z is an integer of 1 to 10)),

m′ is an integer of 1 to 2,300, and

o1′, p1′, q1″, q2″, n1, n2, n3, and n4 are the numbers of repeatingunits,

wherein o1′ is an integer of 0 to 400,

p1′ is an integer of 0 to 400,

q1″ is an integer of 1 to 300,

q2″ is an integer of 0 to 300,

o1 and q2″ are not 0 at the same time,

n1 is an integer of 1 to 2,000,

n2 is an integer of 0 to 2,000,

n3 is an integer of 0 to 2,000, and

n4 is an integer of 0 to 2,000.

In this case, the halogen element may be a fluorine element.

Specifically, in Formula 2, Y′ is an alkyl group having 1 to 10 carbonatoms which is substituted with at least one halogen element, an alkylgroup having 3 to 10 carbon atoms which is unsubstituted with an halogenelement, an alkenyl group having 2 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element,—(CH₂)_(d)—(CR₁₈R₁₉)_(c)—OH (R₁₈ and R₁₉ each are hydrogen or an alkylgroup having 1 to 5 carbon atoms, and d and e are each independently aninteger of 0 to 10), —(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ and R₈ each are asubstituted or unsubstituted alkylene group having 1 to 15 carbon atoms,and g and h each are an integer of 1 to 10), —(CH₂)_(i)—O—CH₂═CH₂ (i isan integer of 0 to 10), or —(CH₂)_(w)—NCO (w is an integer of 1 to 10),and

R_(5a) to R_(5c) are each independently an alkyl group having 1 to 10carbon atoms which is substituted or unsubstituted with at least onehalogen element, an alkenyl group having 2 to 12 carbon atoms which issubstituted or unsubstituted with at least one halogen element, a cyclicalkyl group having 3 to 8 carbon atoms, a heterocyclic group having 3 to8 carbon atoms, a cyclic ether group having 3 to 8 carbon atoms, aheterocyclic ether group having 3 to 8 carbon atoms,—(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ and R₈ each are a substituted orunsubstituted alkylene group having 1 to 10 carbon atoms, and g and heach are an integer of 1 to 10), an aryl group having 6 to 12 carbonatoms, —(CH₂)_(f)—CN (f is an integer of 0 to 10), —(CH₂)_(i)—O—CH₂═CH₂(i is an integer of 0 to 10), —(CH₂)_(j)—Si(R₉)_(k)(OCH₂CH₃)_(3-k) (R₉is H, j is an integer of 1 to 10, and k is an integer of 1 to 3),—(CH₂)_(w)—NCO (w is an integer of 2 to 10), —CH₂CH₂—N(CH₃)₂,—(CH₂)_(x)-A (A=—OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 8) orC(═O)(CH₂)_(z)COOH (z is an integer of 1 to 8)),

m′ is an integer of 1 to 2,000, and

o1′, p1′, q1″, q2″, n1, n2, n3, and n4 are the numbers of repeatingunits,

wherein o1′ is an integer of 0 to 350,

p1′ is an integer of 0 to 350,

q1″ is an integer of 1 to 250,

q2″ is an integer of 0 to 250,

o1′ and q2″ are not 0 at the same time,

n1 is an integer of 1 to 1,500,

n2 is an integer of 0 to 1,500,

n3 is an integer of 0 to 1,500, and

n4 may be an integer of 0 to 1,500.

More specifically, in Formula 2, Y′ is an alkyl group having 1 to 10carbon atoms which is substituted with at least one halogen element, analkyl group having 3 to 10 carbon atoms which is unsubstituted with anhalogen element, an alkenyl group having 2 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element, or—(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH (R₁₈ and R₁₉ each are hydrogen or an alkylgroup having 1 to 5 carbon atoms, and d and e are each independently aninteger of 0 to 10),

R_(5a) is an alkyl group having 1 to 10 carbon atoms which issubstituted or unsubstituted with at least one halogen element, analkenyl group having 2 to 12 carbon atoms which is substituted orunsubstituted with at least one halogen element, a cyclic alkyl grouphaving 3 to 8 carbon atoms, a heterocyclic group having 3 to 8 carbonatoms, a cyclic ether group having 3 to 8 carbon atoms,—(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH (R₁₈ and R₁₉ each are hydrogen or an alkylgroup having 1 to 3 carbon atoms, and d and e are each independently aninteger of 1 to 10), —(CH₂)_(i)—O—CH₂═CH₂ (i is an integer of 1 to 9),—(R₇)_(g)—O—(R₈)_(h)—CH₃ (R₇ and R₈ each are a substituted orunsubstituted alkylene group having 1 to 10 carbon atoms, and g and heach are an integer of 1 to 8), or —(CH₂)_(x)-A (A=—OC(═O)(CH₂)_(y)COOH(y is an integer of 1 to 8) or C(═O)(CH₂)_(z)COOH (z is an integer of 1to 8)),

R_(5b) is an alkyl group having 1 to 10 carbon atoms which issubstituted or unsubstituted with at least one halogen element, analkenyl group having 2 to 12 carbon atoms which is substituted orunsubstituted with at least one halogen element, a heterocyclic grouphaving 3 to 8 carbon atoms, a cyclic ether group having 3 to 8 carbonatoms, a heterocyclic ether group having 3 to 8 carbon atoms,—(CH₂)_(f)—CN (f is an integer of 1 to 8), —(CH₂)_(w)—NCO (w is aninteger of 2 to 10), —CH₂CH₂—N(CH₃)₂, or —(CH₂)_(x)-A(A=—OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 8) orC(═O)(CH₂)_(Z)COOH (z is an integer of 1 to 8)), and

R_(5c) may be a cyclic alkyl group having 3 to 8 carbon atoms, aheterocyclic group having 3 to 8 carbon atoms, a cyclic ether grouphaving 3 to 8 carbon atoms, a heterocyclic ether group having 3 to 8carbon atoms, an aryl group having 6 to 12 carbon atoms, —(CH₂)_(f)—CN(f is an integer of 1 to 8), —(CH₂)_(w)—NCO (w is an integer of 2 to10), or —CH₂CH₂—N(CH₃)₂.

In Formula 2, units having the same or different structure as the unitsderived from the acrylate-based monomer or the acrylate-based polymer,which respectively constitute the repeating unit n1 to the repeatingunit n4, may be further substituted and boned to Y′ or R_(5a) to R_(5c),terminal groups of the units derived from the acrylate-based monomer orthe acrylate-based polymer containing the ion conductive functionalgroup.

In Formula 2, a weight ratio of the repeating unit q1″: the repeatingunit o1′: the repeating unit p1′: the repeating unit q2″ may be in arange of 1:0:0.01:0.01 to 1:100:100:300.

In this case, when the weight ratio of each of the repeating unit o1′and the repeating unit p1′ to the repeating unit q1″ is greater than100, or the weight ratio of the repeating unit q2″ to the repeating unitq1″ is greater than 300, since an effect of reducing the crystallinityof the fluorine-based polymer electrolyte is insignificant, the fluidityof the polymer chain is reduced, and thus, solubility in the electrolytemay be reduced. Also, if the weight ratio of each of the repeating unitp1′ and the repeating unit q2″ to the repeating unit q1″ is less than0.01, since a radical-scavenging effect by the fluorine group isinsignificant, high-temperature, high-voltage stability of the batterymay be reduced.

Furthermore, in Formula 2, a weight ratio of the repeating unit n1: therepeating unit n2: the repeating unit n3: the repeating unit n4 may bein a range of 1:0:0:0 to 1:10:10:10.

In a case in which the repeating units of the units derived from theacrylate-based polymer satisfy the above range, since the wetting to theelectrode may be further improved by controlling the viscosity andsurface energy of the copolymer with respect to the electrolytesolution, high capacity and high energy density of the secondary batterymay be secured.

The copolymer for a polymer electrolyte of the present invention asdescribed above may include at least one selected from the groupconsisting of:

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate) (PVDF-co-P(CTFE-g-P(EGMA)));

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate-r-hydroxybutylacrylate)-co-polytrifluoroethylene(PVDF-co-P(CTFE-g-P(EGMA-r-HBA))-co-PTrFE);

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate-r-butyl acrylate-r-hydroxybutylacrylate))-co-polytrifluoroethylene(PVDF-co-P(CTFE-g-P(EGMA-r-BA-r-HBA))-co-PTrFE);

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate) (P(CTFE-g-P(EGMA)));

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate)-co-polytrifluoroethylene (P(CTFE-g-P(EGMA))-co-PTrFE);

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) trifluoroethoxyethyl methacrylate)(PVDF-co-P(CTFE-g-P(tri-FEGMA));

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) propoxyethyl methacrylate) (PVDF-co-P(CTFE-g-P(PEGMA));

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) propenoxyethyl methacrylate) (PVDF-co-P(CTFE-g-P(PEEGMA));

polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate-r-hydroxybutylacrylate-acryloyloxyethyl isocyanate)-co-polytrifluoroethylene;

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-hydroxybutyl acrylate);

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate);

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate-r-acryloyloxyethylisocyanate);

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-hydroxybutyl acrylate)-co-polytrifluoroethylene;

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutylacrylate)-co-polytrifluoroethylene; and

polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate-r-acryloyloxyethylisocyanate)-co-polytrifluoroethylene.

In the copolymer for a polymer electrolyte represented by Formula 1 andFormula 2, a moiety (—O—(CR—CH₂O)_(x)—) contained in the unit derivedfrom the acrylate-based monomer or the acrylate-based polymer may beincluded in an amount of 1 wt % to 90 wt %, for example, 10 wt % to 60wt % based on a total weight of the copolymer for a polymer electrolyte.

If the amount of the moiety is less than 1 wt %, since electrode wettingis not sufficient due to an insignificant effect of improving surfaceenergy of the copolymer for a polymer electrolyte, penetration of theelectrolyte solution into the electrode is not easy. Thus, it isdifficult to form a stable SEI on the surface of the electrode activematerial. In contrast, if the amount of the moiety is greater than 90 wt%, since the radical-scavenging effect by the fluorine-based polymer isinsignificant, desired purpose of improving high-temperature stabilityof the battery may not be achieved.

With respect to the copolymer for a polymer electrolyte of the presentinvention, the structure and ratio of the unit derived from theacrylate-based monomer or the acrylate-based polymer containing the ionconductive functional group grafted to the main chain structure or mainchain containing a fluorine element may be adjusted.

For example, in Formula 1 and Formula 2, a weight ratio of thefluorine-based polymer main chain to the unit derived from theacrylate-based monomer or the acrylate-based polymer (e.g., unit n inFormula 1 or units n1 to n4 in Formula 2), as the side chain, may be ina range of 1:99 to 40:60, preferably 2:98 to 30:70, and moreparticularly 5:95 to 25:75.

If, in Formula 1 and Formula 2, in a case in which the weight ratio ofthe unit derived from the acrylate-based monomer or the acrylate-basedpolymer containing the ion conductive functional group is greater than99, since the radical-scavenging effect by the fluorine-based polymer isinsignificant, the desired purpose of improving the high-temperaturestability of the battery may not be achieved. Furthermore, in a case inwhich the weight ratio of the unit derived from the acrylate-basedmonomer or the acrylate-based polymer containing the ion conductivefunctional group is less than 60, the solubility of the copolymer for apolymer electrolyte in the solution of the composition for a polymerelectrolyte is reduced while the surface energy of the copolymer for apolymer electrolyte is reduced. That is, since affinity between theelectrolyte solution and the copolymer is low at room temperature due toa difference in polarity, an effect of improving miscibility between theelectrolyte solution and the copolymer is insignificant. Thus, it isdifficult to prepare an injection-type electrolyte solution during thepreparation of the lithium secondary battery, and, as a result, sincethe penetration of the copolymer for a polymer electrolyte into theelectrode is not easy, the electrode wetting is reduced. As a result,since the copolymer for a polymer electrolyte is difficult to beuniformly distributed on the surface of the electrode, there is adisadvantage in that it is difficult to form a stable SEI on the surfaceof the electrode.

Also, a weight-average molecular weight of the copolymer for a polymerelectrolyte of the present invention may be adjusted according to themain-chain structure containing a fluorine element or the structure andratio of the acrylate-based moiety of the side chain, and mayspecifically be in a range of 1,000 g/mol to 200,000 g/mol.

In a case in which the weight-average molecular weight of the copolymerfor a polymer electrolyte is within the above range, ion transfercapability of the polymer electrolyte is improved, and electrochemicalstability may be secured.

For example, if the weight-average molecular weight of the copolymer fora polymer electrolyte is greater than 200,000 g/mol, since thesolubility in the composition for a polymer electrolyte is reduced toincrease the viscosity of the composition for a polymer electrolyte,even if a small amount of the copolymer for a polymer electrolyte isadded, the viscosity exceeds a predetermined level to reduce wetting andimpregnability in the battery, and thus, the electrochemical stabilityof the secondary battery may be reduced.

The weight-average molecular weight (Mw) of the copolymer for a polymerelectrolyte may be measured using gel permeation chromatography (GPC).For example, a sample having a predetermined concentration is prepared,and Alliance 4, a GPC measurement system, is then stabilized. When thesystem is stabilized, a standard sample and the sample are injected intothe system to obtain a chromatogram, and a molecular weight may then becalculated using an analytical method (system: Alliance 4, column:Ultrahydrogel linearX2, eluent: 0.1M NaNO₃ (pH 7.0 phosphate buffer,flow rate: 0.1 mL/min, temp: 40° C., injection: 100 μL)).

With respect to the copolymer for a polymer electrolyte of the presentinvention as described above, the surface energy may be improved and thesolubility in the electrolyte solution may be improved by theacrylate-based functional group containing the ion conductive functionalgroup grafted to the side chain of the fluorine-based polymer. Thus, thecopolymer for a polymer electrolyte does not cause an increase ininternal resistance of the electrode even if the copolymer for a polymerelectrolyte containing the fluorine-based element is introduced into theelectrolyte, and may not only maintain a state in which it is uniformlydistributed in the electrolyte, but may also exhibit theradical-scavenging action by the fluorine element and an improvement inthe high-temperature stability of the positive electrode.

Method of Preparing Polymer for Electrolyte

Hereinafter, a method for preparing the copolymer for a polymerelectrolyte of the present invention will be described.

The copolymer for a polymer electrolyte of the present invention may beprepared by a graft-from atomic transfer radical polymerization(hereinafter, referred to as “ATRP”) method for preparing a high-puritypolymer by controlling a molecular weight distribution.

The graft-from ATRP method may include the steps of:

(a) mixing an acrylate-based monomer or an acrylate-based polymer with afluorine-based polymer, as a polymerizable monomer, in a reactionsolvent;

(b) adding an ATRP catalyst into the reaction solvent and performing anATRP reaction to prepare a copolymer for a polymer electrolyte; and

(c) adding an appropriate non-solvent to remove the catalyst and theunreacted monomers.

Various solvents known in the art may be used as the reaction solvent,and, for example, N-methyl-2-pyrrolidone (NMP), gamma-butyrolactone(GBL), dimethylformamide (DMF), dimethyl sulfoxide (DMSO),dimethylacetamide (DMAc), acetonitrile (AcCN), ethyl acetate (EA),methyl ethyl ketone (MEK), or tetrahydrofuran (THF) may be used, but thereaction solvent is not limited thereto.

Also, the fluorine-based polymer and the acrylate-based monomer or theacrylate-based polymer containing a lithium ion conductive group may bemixed in a weight ratio of 1:0.1 to 1:99.

If the acrylate-based monomer or acrylate-based polymer is mixed in aweight ratio of less than 0.1, since solubility of the polymer in thesolvent is reduced during the preparation of the composition for apolymer electrolyte of the present invention to be described later andionic conductivity is excessively reduced, there is a risk that internalresistance of the battery is increased. In contrast, if theacrylate-based monomer is mixed in a weight ratio of greater than 99,since the radical-scavenging effect by the fluorine element isinsignificant, the desired purpose of improving the high-temperaturestability of the battery may not be achieved.

Furthermore, the catalyst is a catalyst usable for the ATRP reaction,wherein typical examples thereof may be Cu(I)Cl, Cu(II)Cl₂, Cu(I)Br,Cu(II)Br₂, Fe(II)Cl₂, Fe(III)Cl₃, or a mixture thereof, but the catalystmay preferably include Cu(I)Cl, Cu(II)Cl₂, Cu(I)Br, Cu(II)Br₂, or amixture thereof.

The catalyst may be included in an amount of 0.0001 part by weight to 1part by weight, 0.0005 part by weight to 0.5 part by weight, or 0.001part by weight to 0.1 part by weight based on 100 parts by weight of thetotal monomer mixture. In a case in which the amount of the catalyst isless than 0.0001 part by weight, a reaction rate is very slow, and, in acase in which the amount of the catalyst is greater than 1 part byweight, since gelation may occur before the formation of the polymer orthe removal of the catalyst may be very difficult, the amount of thecatalyst is appropriately selected within the above range.

A ligand or a reducing agent may be further mixed with the catalystduring the ATRP reaction, if necessary.

The ligand is not particularly limited as long as it is bonded to thecatalyst to be able to be used in the polymerization reaction, whereinthe ligand may be exemplified by, for example, a ligand having at leastone nitrogen, oxygen, phosphorus, and sulfur atom capable ofcoordinating with the catalyst through a 6-bond or a ligand containingtwo or more carbon atoms capable of coordinating with the catalystthrough a n-bond, but the ligand is not limited thereto. Specifically,the ligand may include at least one selected from the group consistingof PMDETA (N,N,N′,N″,N′″-pentamethyldiethylenetriamine), bpy(2,2′-bipyridine), dNbpy (4,4′-di-5-nonyl-2,2′-bipyridine), TPMA(tris(2-pyridylmethyl)amine), and Me6TREN(tris(2-dimethylaminoethyl)amine).

The ligand may be included in an amount of 50 parts by weight to 2,000parts by weight, 100 parts by weight to 1,000 parts by weight, or 200parts by weight to 500 parts by weight based on 100 parts by weight ofthe catalyst. In a case in which the amount of the ligand is less than50 parts by weight, since the formation of a metal composite by bondingwith the catalyst is excessively small, the reaction is very slow ordoes not proceed, and, in a case in which the amount of the ligand isgreater than 2,000 parts by weight, manufacturing cost is increased andthere is a concern that a side reaction due to the use of the excessiveamount of the ligand may occur.

An example of the reducing agent may be a radical generator, such asazobisisobutyronitrile (AIBN), an organic reducing agent, or aninorganic reducing agent, which is commonly used in the ATRP reaction,but the reducing agent is not limited thereto.

The ATRP method of the present invention is performed under atemperature condition of 30° C. to 100° C., for example, 60° C., and mayobtain a copolymer for a polymer electrolyte having a controlledstructure or molecular weight.

Since electrons are balanced and decomposed between activated carbon andthe halogen element (Cl) of the fluorine-based polymer, such aspolychlorotrifluoroethylene (PCTFE), by the metal catalyst, such asCu(I)Cl, during the ATRP reaction, a radical with properties that do notlose polymerization activity (e.g., living properties) is formed at anend of the fluorine-based polymer. Since the polymerization reaction isinitiated by the radical, the unit derived from the acrylate-basedmonomer or the acrylate-based polymer containing the ion conductivefunctional group may be grafted to the main chain of the fluorine-basedpolymer as a pendant group.

In addition to the graft-from ATRP method, the copolymer for a polymerelectrolyte of the present invention may also be synthesized by a livingpolymerization method such as an ATRP polymerization method using aninitiator containing a nucleophilic functional group or a ReversibleAddition-Fragmentation Chain Transfer (RAFT) polymerization method.

The ATRP polymerization method using the initiator containing thenucleophilic functional group is a method in which, after a polymer fora side chain containing a hydroxy group at its end is synthesized byusing an ATRP initiator containing a hydroxy group, the polymer for aside chain is reacted with a fluorine-based polymer to be introduced asa pendant group. In this case, the nucleophilic functional group may besubstituted into a position of chlorine (Cl) element contained in thefluorine-based polymer in the presence of a reducing agent, such as LiH,NaH, and LiBH₄, to form a branched polymer.

Also, the RAFT method is a method in which, after a terminal RAFTfunctional group is reduced to synthesize a polymer for a side chaincontaining a thiol group, the polymer for a side chain is reacted with afluorine-based polymer to be introduced as a pendant group.

Composition for Polymer Electrolyte

Also, in the present invention, provided is a composition for a polymerelectrolyte which includes (1) a non-aqueous electrolyte solution and(2) the copolymer for a polymer electrolyte of the present invention.

(1) Non-Aqueous Electrolyte Solution

The non-aqueous electrolyte solution is an electrolyte solution used ina conventional lithium secondary battery, wherein the non-aqueouselectrolyte solution is not particularly limited as long as it includesa lithium salt and an organic solvent.

(1-1) Lithium Salt

The lithium salt is used as an electrolyte salt in the lithium secondarybattery, wherein it is used as a medium for transferring ions.Typically, the lithium salt may include at least one compound selectedfrom the group consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄,LiN(C₂FsSO₂)₂, LiN(CF₃SO₂)₂, CF₃SO₃Li, LiC(CF₃SO₂)₃, LiC₄BO₃, LiTFSI,LiFSI, and LiClO₄, and may preferably include LiPF₆, but the lithiumsalt is not limited thereto.

One or a mixture of two or more thereof, if necessary, may be used asthe lithium salt. The lithium salt may be included in an amount of 0.5 Mto 5 M, for example, 0.5 M to 4 M. In a case in which the amount of thelithium salt is less than the above range, since a concentration oflithium ions in the electrolyte is low, charge and discharge of thebattery may not be performed properly, and, in a case in which theamount of the lithium salt is greater than the above range, sinceviscosity of the gel polymer electrolyte may be increased to reducewetting in the battery, performance of the battery may be degraded.

(1-2) Organic Solvent

The organic solvent is an electrolyte solution solvent typically used ina lithium secondary battery, wherein a solvent, which may minimizedecomposition due to an oxidation reaction during charge and dischargeof the secondary battery and may exhibit desired characteristics with anadditive, may be used as the organic solvent.

A cyclic carbonate organic solvent, a linear carbonate organic solvent,and a mixed organic solvent thereof may be used as the organic solvent,and, specifically, in order to improve charge and discharge performanceof the battery, high ionic conductivity and high permittivity may beachieved by mixing the cyclic carbonate-based organic solvent and thelinear carbonate-based organic solvent.

The cyclic carbonate organic solvent is a solvent which well dissociatesthe lithium salt in the electrolyte due to high permittivity as a highlyviscous organic solvent, wherein the cyclic carbonate organic solventmay specifically include at least one organic solvent selected from thegroup consisting of ethylene carbonate (EC), propylene carbonate (PC),1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate,2,3-pentylene carbonate, vinylene carbonate, and fluoroethylenecarbonate (FEC), and, among them, the cyclic carbonate organic solventmay include ethylene carbonate or propylene carbonate which may stablymaintain passivation ability of the SEI.

The linear carbonate organic solvent is an organic solvent having lowviscosity and low permittivity, wherein the linear carbonate organicsolvent may include at least one organic solvent selected from the groupconsisting of dimethyl carbonate (DMC), diethyl carbonate (DEC),dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate,and ethylpropyl carbonate, and, among them, the linear carbonate organicsolvent may include dimethyl carbonate (DMC) having low viscositycharacteristics while having a small molecular size.

In addition, at least one selected from a cyclic ester organic solvent,a linear ester organic solvent, an ether organic solvent, a glymeorganic solvent, and a nitrile organic solvent may be further used asthe organic solvent, if necessary.

Specific examples of the cyclic ester organic solvent may be at leastone organic solvent selected from the group consisting ofγ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, andε-caprolactone.

The linear ester organic solvent may include at least one organicsolvent selected from the group consisting of methyl acetate, ethylacetate, propyl acetate, methyl propionate, ethyl propionate, propylpropionate, and butyl propionate.

As the ether organic solvent, any one selected from the group consistingof dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether,methylpropyl ether, and ethylpropyl ether or a mixture of two or morethereof may be used.

The glyme organic solvent is a solvent having higher dielectric constantand lower surface tension than the linear carbonate organic solvent andhaving lower reactivity with metal, wherein the glyme organic solventmay include at least one selected from the group consisting ofdimethoxyethane (glyme, DME), diglyme, triglyme, and tetraglyme(TEGDME), but the glyme organic solvent is not limited thereto.

The nitrile organic solvent may include at least one selected from thegroup consisting of acetonitrile, propionitrile, butyronitrile,valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile,4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile,phenylacetonitrile, 2-fluorophenylacetonitrile, and4-fluorophenylacetonitrile, but the nitrile organic solvent is notlimited thereto.

(2) Copolymer for Polymer Electrolyte

Since a description of the copolymer for a polymer electrolyte, which isincluded in the composition for a polymer electrolyte of the presentinvention, overlaps with those described above, the description thereofwill be omitted.

However, the amount of the copolymer for a polymer electrolyte in theelectrolyte composition may be appropriately selected in considerationof the weight-average molecular weight of the copolymer for a polymerelectrolyte so as to achieve impregnability, SEI-forming effect, andproperties, such as viscosity and ionic conductivity, of the electrolytecomposition, and the copolymer for a polymer electrolyte mayspecifically be included in an amount of 0.01 wt % to 30 wt % based on atotal weight of the composition for a polymer electrolyte.

Specifically, in a case in which the weight-average molecular weight ofthe copolymer for a polymer electrolyte of the present invention is150,000 g/mol or more, the copolymer for a polymer electrolyte may beincluded in an amount of 0.01 wt % to 20 wt %, particularly 0.1 wt % to20 wt %, and more particularly 0.1 wt % to 18 wt % based on the totalweight of the composition for a polymer electrolyte, and, in a case inwhich the weight-average molecular weight of the copolymer for a polymerelectrolyte is less than 150,000 g/mol, the copolymer for a polymerelectrolyte may be included in an amount of 0.01 wt % to 30 wt %, forexample, 1 wt % to 30 wt % based on the total weight of the compositionfor a polymer electrolyte.

That is, when the weight-average molecular weight of the copolymer for apolymer electrolyte is about 160,000 g/mol, if the copolymer for apolymer electrolyte is included in an amount of 20 wt % or more based onthe total weight of the composition for a polymer electrolyte, an effectof reducing surface tension due to structural characteristics of thepolymer may occur, but safety of the battery may be reduced as ionicconductivity is reduced due to an increase in viscosity of thecomposition for a polymer electrolyte.

In a case in which the copolymer for a polymer electrolyte is includedwithin the above range, the impregnability, SEI-forming effect, andphysical properties, such as viscosity and ionic conductivity, of thecomposition for a polymer electrolyte may be secured. If the copolymerfor a polymer electrolyte is included in an amount of less than 0.01 wt%, since a network reaction between polymers for forming a gel polymerelectrolyte is difficult to be formed, a stability improvement effectdue to the introduction of the gel polymer electrolyte of the presentinvention may be insignificant. Also, in a case in which the copolymerfor a polymer electrolyte is included in an amount of greater than 30 wt%, the safety of the battery may be reduced as the ionic conductivity isreduced due to the increase in the viscosity of the composition for apolymer electrolyte.

(3) Polymerization Initiator

The electrolyte composition of the present invention may further includea polymerization initiator.

The polymerization initiator is to form a polymer network bonded in athree-dimensional structure by polymerizing the polymer of the presentinvention, wherein a conventional polymerization initiator known in theart may be used without limitation. The polymerization initiator mayinclude a photopolymerization initiator or a thermal polymerizationinitiator according to a polymerization method.

Specifically, as a representative example, the photopolymerizationinitiator may include at least one selected from the group consisting of2-hydroxy-2-methylpropiophenone (HMPP),1-hydroxy-cyclohexylphenyl-ketone, benzophenone,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone,oxy-phenylacetic acid 2-[2-oxo-2-phenyl-acetoxy-ethoxy]-ethyl ester,oxy-phenyl-acetic 2-[2-hydroxyethoxy]-ethyl ester,alpha-dimethoxy-alpha-phenylacetophenone,2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone,2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone,diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis(eta5-2,4-cyclopentadiene-1-yl), bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium, 4-isobutylphenyl-4′-methylphenyl iodonium,hexafluorophosphate, and methyl benzoylformate.

Also, as a representative example, the thermal polymerization initiatormay include at least one selected from the group consisting of benzoylperoxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide,t-butyl peroxy-2-ethyl-hexanoate, cumyl hydroperoxide, hydrogenperoxide, 2,2′-azobis(2-cyanobutane), 2,2′-azobis(methylbutyronitrile),2,2′-azobis(iso-butyronitrile) (AIBN), and2,2′-azobisdimethyl-valeronitrile (AMVN).

The polymerization initiator forms a radical by being dissociated byheat at 30° C. to 100° C. in the battery or by being dissociated bylight, such as ultraviolet (UV), at room temperature (5° C. to 30° C.),and forms cross-linking by free radical polymerization so that a polymermay be polymerized.

The polymerization initiator may be used in an amount of 0.01 part byweight to 5 parts by weight, preferably 0.05 part by weight to 5 partsby weight, and more preferably 0.1 part by weight to 5 parts by weightbased on 100 parts by weight of the copolymer for a polymer electrolyte.If the polymerization initiator is used within the above range, anamount of the unreacted polymerization initiator, which may adverselyaffect the battery performance, may be minimized. Also, in a case inwhich the polymerization initiator is included within the above range,gelation may be performed properly.

The electrolyte composition of the present invention may not include apolymerization initiator in order to provide a non-crosslinkeddispersion-type gel polymer electrolyte in which cross-linking betweenthe polymers for an electrolyte of the present invention is not formed.

(4) Multifunctional Crosslinking Agent

Also, the electrolyte composition of the present invention may furtherinclude a multifunctional crosslinking agent in order to induce acrosslinking reaction between the monomers and simultaneously furtherimprove the high-temperature stability of the positive electrode.

The multifunctional crosslinking agent may react with a curingfunctional group included in the fluorine-based linear copolymer to forma crosslinked structure between polymers. Since an electrode protectivelayer formed in the crosslinked structure exhibits high chemical andelectrochemical stability and protects the surface of the electrodeactive material from a side reaction with the electrolyte, problems,such as a decrease in Coulombic efficiency and degradation of cyclecharacteristics of the secondary battery, may be overcome.

A type of the multifunctional crosslinking agent is not particularlylimited, and any one selected from the group consisting of an isocyanatecrosslinking agent, an epoxy crosslinking agent, an aziridinecrosslinking agent, an alcohol crosslinking agent, an amine-basedcrosslinking agent, and a vinyl-based crosslinking agent may be used.

As specific examples of the isocyanate crosslinking agent, adiisocyanate compound, such as toluene diisocyanate, xylenediisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate,isoborone diisocyanate, tetramethylxylene diisocyanate, or naphthalenediisocyanate, or a compound obtained by reacting the diisocyanatecompound with a polyol may be used, and, as the polyol, for example,trimethylol propane may be used.

Specific examples of the epoxy crosslinking agent may be at least oneselected from the group consisting of ethylene glycol diglycidyl ether,triglycidyl ether, trimethylolpropane triglycidyl ether,N,N,N′,N′-tetraglycidyl ethylenediamine, and glycerin diglycidyl ether.

Specific examples of the aziridine crosslinking agent may be at leastone selected from the group consisting ofN,N′-toluene-2,4-bis(1-aziridinecarboxamide),N,N′-diphenylmethane-4,4′-bis(1-aziridinecarboxamide), triethylenemelamine, bisisoprotaloyl-1-(2-methylaziridine), andtri-1-aziridinylphosphine oxide, but the aziridine crosslinking agent isnot limited thereto.

Specific examples of the alcohol crosslinking agent may be at least oneselected from the group consisting of poly(alkylene glycol), glycerol,trismethylol propane, pentaerythritol, and dipentaerythritol, but thealcohol crosslinking agent is not limited thereto.

Also, specific examples of the amine-based crosslinking agent may be atleast one selected from the group consisting of ethylenediamine,diethylenetriamine, triethylenetetramine, or modified amines thereof,and metaphenylenediamine, diaminodiphenylmethane, diaminodiphenylsulfone, or modified amines thereof, but the amine-based crosslinkingagent is not limited thereto.

The vinyl-based crosslinking agent is an organic compound having two ormore vinyl groups in one molecule, wherein the vinyl-based crosslinkingagent may include at least one selected from ethylene glycoldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, tri(propylene glycol)di(meth)acrylate, tris(2-(meth)acryloethyl) isocyanate,trimethylolpropane tri(meth)acrylate, trimethylolpropane ethoxylatetri(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritoltri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol di(meth)acrylate, dipentaerythritol tri(meth)acrylate,dipentaerythritol tetra(meth)acrylate, dipentaerythritolpenta(meth)acrylate, and dipentaerythritol hexa(meth)acrylate, but thevinyl-based crosslinking agent is not limited thereto.

The multifunctional crosslinking agent may be included in an amount of 1part by weight to 1,000 parts by weight, for example, 5 parts by weightto 500 parts by weight based on 100 parts by weight of thefluorine-based graft polymer. Physical properties of the electrolyte maybe appropriately expressed at a desired level by controlling the amountof the crosslinking agent within the above-described range.

If, in a case in which the amount of the multifunctional crosslinkingagent is less than 1 part by weight, since the amount is not sufficientto react the polymer, the polymer polymerization reaction may not becaused. In contrast, in a case in which the amount of themultifunctional crosslinking agent is greater than 1,000 parts byweight, since reactivity of the crosslinking agent is excessive, it isnot easy to control the weight-average molecular weight of the polymerfor a gel polymer electrolyte.

(5) Other Additives

Also, in order to further achieve effects of improving high-temperaturestorage characteristics, cycle life characteristics, low-temperaturehigh rate discharge characteristics, overcharge prevention, andhigh-temperature swelling, the electrolyte composition of the presentinvention may further include other additives, if necessary.

These other additives are not particularly limited as long as these areadditives capable of forming a stable film on surfaces of positiveelectrode and negative electrode while not significantly increasinginitial resistance.

These other additives may include a commonly known electrolyte solutionadditive, specifically, at least one selected from the group consistingof vinylene carbonate (VC), LiBF₄, vinylethylene carbonate (VEC),1,3-propane sultone (PS), 1,3-propene sultone (PRS), succinonitrile(SN), adiponitrile (Adn), fluoroethylene carbonate (FEC), ethylenesulfate (Esa), LiPO₂F₂, methyl trimethylene sulfate (MTMS), LiODFB(lithium difluorooxalatoborate), LiBOB (lithium bis-(oxalato)borate),tetraphenylborate (TPB), TMSPa (3-trimethoxysilanyl-propyl-N-aniline),TMSPi (tris(trimethylsilyl)phosphite),tris(2,2,2-trifluoroethyl)phosphate (TFEPa), andtris(trifluoroethyl)phosphite (TFEPi).

It is known that, particularly, vinylene carbonate, LiBF₄, 1,3-propanesultone (PS), and ethylene sulfate (Esa), among these other additives,may form a stable SEI on the surface of the negative electrode during aninitial activation process of the secondary battery.

The other additives may be included in an amount of 10 wt % or less, forexample, 0.5 wt % to 7 wt % based on the total weight of the electrolytecomposition. If the amount of the other additives is greater than 10 wt%, not only there is a possibility that a side reaction in theelectrolyte solution occurs excessively during charge and discharge dueto the excessive amount of the additive used, but the additives may alsobe present in the form of an unreacted material or precipitates in theelectrolyte solution at room temperature because the additives may notbe sufficiently decomposed at high temperatures, and, accordingly, lifeor resistance characteristics of the secondary battery may be degraded.

Furthermore, the electrolyte composition of the present invention mayinclude inorganic particles as other additives. Also, as the inorganicparticles, a single compound selected from the group consisting ofBaTiO₃, Pb(Zr,Ti)O₃ (PZT), Pb_(1-a)La_(a)Zr_(1-b)Ti_(b)O₃ (PLZT, where0<a<1, 0<b<1), Pb(Mg_(1/3)Nb_(2/3))O₃—PbTiO₃ (PMN-PT), hafnia (HfO₂),SrTiO₃, SnO₂, CeO₂, MgO, NiO, CaO, ZnO, ZrO₂, Y₂O₃, Al₂O₃, Al(OH)₃,TiO₂, SiO₂, and SiC, or a mixture of at least two thereof may be used.In addition, inorganic particles having lithium ion transfer capability,that is, lithium phosphate (Li₃PO₄), lithium titanium phosphate(Li_(c)Ti_(d)(PO₄)₃, 0<c<2, 0<d<3), lithium aluminum titanium phosphate(Li_(a1)Al_(b1)Ti_(c1)(PO₄)₃, 0<a1<2, 0<b1<1, 0<c1<3),(LiAlTiP)_(a2)O_(b2)-based glass (0<a2<4, 0<b2<13) such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅, lithium lanthanum titanate(Li_(a3)L_(a3)TiO₃, 0<a3<2, 0<b3<3), lithium germanium thiophosphate(Li_(a4)Ge_(b4)P_(c2)S_(d), 0<a4<4, 0<b4<1, 0<c2<1, 0<d<5) such asLi_(3.25)Ge_(0.25)P_(0.75)S₄, lithium nitride (Li_(a5)N_(b5), 0<a5<4,0<b5<2) such as Li3N, SiS2-based glass (Lia6Sib6Sc3, 0<a6<3, 0<b6<2,0<c3<4) such as Li₃PO₄—Li₂S—SiS₂, P₂S₅-based glass (Li_(a7)P_(b7)S_(c5),0<a7<3, 0<b7<3, 0<c5<7), such as LiI—Li₂S—P₂S₅, or a mixture thereof maybe used.

The inorganic particles may be included in an amount of 10 wt % or less,for example, 0.5 wt % to 7 wt % based on the total weight of theelectrolyte composition. If the amount of the other additives is greaterthan 10 wt %, capacity characteristics and life characteristics of thesecondary battery may be degraded due to an increase in resistancecaused by the excessive amount of the additive used.

The composition for a polymer electrolyte of the present invention,which is prepared according to an embodiment of the present invention,may have a viscosity of 3 cp to 20 cp or less at room temperature (25°C.). If the viscosity is greater than the above range, bubbles may begenerated when the electrolyte is injected or the wetting to theelectrode may be reduced.

Gel Polymer Electrolyte

Also, in the present invention, provided is a gel polymer electrolytewhich is formed by thermal polymerization of the composition for apolymer electrolyte of the present invention.

The gel polymer electrolyte according to the present invention may beformed by polymerization of the composition for a polymer electrolyteaccording to a method by typical in-situ polymerization or coatingpolymerization known in the art, or may be formed by using anddispersing a polymer in the electrolyte.

Specifically, the in-situ polymerization is a method of preparing a gelpolymer electrolyte through steps of (a) inserting an electrode assemblycomposed of a positive electrode, a negative electrode, and a separatordisposed between the positive electrode and the negative electrode intoa battery case, and (b) injecting the composition for a polymerelectrolyte according to the present invention into the battery case andperforming polymerization.

An in-situ polymerization reaction in the lithium secondary battery maybe performed by using E-beam, γ-ray, and room temperature/hightemperature aging processes, and may be performed by thermalpolymerization or photopolymerization. In this case, polymerization timerequired may be in a range of about 2 minutes to about 24 hours, thermalpolymerization temperature may be in a range of 30° C. to 100° C., andphotopolymerization temperature may be room temperature (5° C. to 30°C.).

More specifically, the in-situ polymerization reaction in the lithiumsecondary battery forms a gel polymer electrolyte in the form in whichthe gel polymer electrolyte composition is injected into a battery celland then subjected to gelation through the polymerization reaction, orforms a gel polymer electrolyte in the form of being dispersed withoutthe gelation through the polymerization reaction.

As another method, after the gel polymer electrolyte composition iscoated on one surface of an electrode and a separator and is cured(gelated) by using heat or light such as UV, an electrode assembly isprepared by winding or laminating the electrode and/or separator havinga gel polymer electrolyte formed thereon, and a lithium secondarybattery may be prepared by inserting the electrode assembly into abattery case and re-injecting a conventional liquid electrolytethereinto.

A conventional gel polymer electrolyte is disadvantageous in that itsionic conductivity is lower than that of a liquid electrolyte andwetting to the electrode is low. Also, stability and mechanicalproperties may be relatively poor in comparison to those of a solidpolymer electrolyte.

However, since the gel polymer electrolyte of the present inventionincludes the copolymer for a polymer electrolyte in which the solubilityin the electrolyte solution is improved by grafting the unit derivedfrom the acrylate-based monomer or the acrylate-based polymer containingthe ion conductive functional group on the fluorine-based polymer, thecopolymer for a polymer electrolyte may form a network in theelectrolyte or may be dispersed in the electrolyte to improve thelithium ion transfer capability and the wetting to the electrode throughphysical/chemical bonding and to improve the mechanical properties.

Thus, a stable SEI may be formed on the surface of the electrode withoutcausing the internal resistance of the electrode, and a gel polymerelectrolyte, in which the radical is easily scavenged by a fluorineatom, may be achieved. Furthermore, a lithium secondary battery havingimproved high-temperature stability may be prepared by including the gelpolymer electrolyte.

Lithium Secondary Battery

Furthermore, in the present invention, a lithium secondary batteryincluding the gel polymer electrolyte may be provided.

Specifically, the lithium secondary battery includes a positiveelectrode including a positive electrode active material, a negativeelectrode including a negative electrode active material, a separatordisposed between the positive electrode and the negative electrode, andthe above-described electrolyte.

In this case, the lithium secondary battery of the present invention maybe prepared according to a conventional method known in the art. Forexample, after an electrode assembly is formed by sequentially stackinga positive electrode, a negative electrode, and a separator disposedbetween the positive electrode and the negative electrode, the lithiumsecondary battery of the present invention may be prepared by insertingthe electrode assembly into a battery case, injecting the compositionfor a gel polymer electrolyte according to the present invention, andthen performing in-situ polymerization.

In this case, since the gel polymer electrolyte is the same as describedabove, a detailed description thereof will be omitted.

(1) Positive Electrode

The positive electrode may be prepared by coating a positive electrodecollector with a positive electrode material mixture slurry including apositive electrode active material, a binder, a conductive agent, and asolvent.

The positive electrode collector is not particularly limited so long asit has conductivity without causing adverse chemical changes in thebattery, and, for example, stainless steel, aluminum, nickel, titanium,fired carbon, or aluminum or stainless steel that is surface-treatedwith one of carbon, nickel, titanium, silver, or the like may be used.

The positive electrode active material is a compound capable ofreversibly intercalating and deintercalating lithium, wherein thepositive electrode active material may specifically include a lithiumcomposite metal oxide including lithium and at least one metal such ascobalt, manganese, nickel, or aluminum. More specifically, the lithiumcomposite metal oxide may include lithium-manganese-based oxide (e.g.,LiMnO₂, LiMn₂O₄, etc.), lithium-cobalt-based oxide (e.g., LiCoO₂, etc.),lithium-nickel-based oxide (e.g., LiNiO₂, etc.),lithium-nickel-manganese-based oxide (e.g., LiNi_(1-Y)Mn_(Y)O₂ (where0<Y<1), LiMn_(2-Z)Ni_(z)O₄ (where 0<Z<2), etc.),lithium-nickel-cobalt-based oxide (e.g., LiNi_(1-Y1)Co_(Y1)O₂ (where0<Y1<1), etc.), lithium-manganese-cobalt-based oxide (e.g.,LiCo_(1-Y2)Mn_(Y2)O₂ (where 0<Y2<1), LiMn_(2-Z1)CO_(z1)O₄ (where0<Z1<2), etc.), lithium-nickel-manganese-cobalt-based oxide (e.g.,Li(Ni_(p)Co_(q)Mn_(r1))O₂ (where 0<p<1, 0<q<1, 0<r1<1, and p+q+r1=1) orLi(Ni_(p1)Co_(q1)Mn_(r2))O₄ (where 0<p1<2, 0<q1<2, 0<r2<2, andp1+q1+r2=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide(e.g., Li(Ni_(p2)Co_(q2)Mn_(r3)M_(S2))O₂ (where M is selected from thegroup consisting of aluminum (Al), iron (Fe), vanadium (V), chromium(Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo),and p2, q2, r3, and s2 are atomic fractions of each independentelements, wherein 0<p2<1, 0<q2<1, 0<r3<1, 0<S2<1, and p2+q2+r3+S2=1),etc.), and any one thereof or a compound of two or more thereof may beincluded.

Among these materials, in terms of the improvement of capacitycharacteristics and stability of the battery, the lithium compositemetal oxide may include LiCOO₂, LiMnO₂, LiNiO₂, lithium nickel manganesecobalt oxide (e.g., Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂,Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂, Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂,Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, and Li(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂),or lithium nickel cobalt aluminum oxide (e.g.,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, etc.), and, in consideration of asignificant improvement due to the control of type and content ratio ofelements constituting the lithium composite metal oxide, the lithiumcomposite metal oxide may include Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, and any one thereof or a mixture of twoor more thereof may be used.

The positive electrode active material may be included in an amount of60 wt % to 99 wt %, preferably 70 wt % to 99 wt %, and more preferably80 wt % to 98 wt % based on a total weight of solids excluding thesolvent in the positive electrode material mixture slurry.

The binder is a component that assists in the binding between the activematerial and the conductive agent and in the binding with the currentcollector.

Examples of the binder may be polyvinylidene fluoride, polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene,polyethylene (PE), polypropylene, an ethylene-propylene-diene monomer, asulfonated ethylene-propylene-diene monomer, a styrene-butadiene rubber,a fluoro rubber, various copolymers, and the like.

The binder may be commonly included in an amount of 1 wt % to 20 wt %,preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %based on the total weight of solids excluding the solvent in thepositive electrode material mixture slurry.

The conductive agent is a component for further improving theconductivity of the positive electrode active material.

The conductive agent is not particularly limited as long as it hasconductivity without causing adverse chemical changes in the battery,and, for example, a conductive material, such as: graphite; acarbon-based material such as carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, and thermal black;conductive fibers such as carbon fibers or metal fibers; metal powdersuch as fluorocarbon powder, aluminum powder, and nickel powder;conductive whiskers such as zinc oxide whiskers and potassium titanatewhiskers; conductive metal oxide such as titanium oxide; orpolyphenylene derivatives, may be used.

The conductive agent may be commonly included in an amount of 1 wt % to20 wt %, preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10wt % based on the total weight of solids excluding the solvent in thepositive electrode material mixture slurry.

The solvent may include an organic solvent, such asN-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the positive electrode activematerial as well as optionally the binder and the conductive agent areincluded. For example, the solvent may be included in an amount suchthat a concentration of a solid content including the positive electrodeactive material as well as optionally the binder and the conductiveagent is in a range of 50 wt % to 95 wt %, preferably 70 wt % to 95 wt%, and more preferably 70 wt % to 90 wt %.

(2) Negative Electrode

The negative electrode, for example, may be prepared by coating anegative electrode collector with a negative electrode material mixtureslurry including a negative electrode active material, a binder, aconductive agent, and a solvent.

For example, in a case in which the negative electrode is prepared bycoating the negative electrode collector with the negative electrodematerial mixture slurry, the negative electrode collector generally hasa thickness of 3 μm to 500 μm. The negative electrode collector is notparticularly limited so long as it has high conductivity without causingadverse chemical changes in the battery, and, for example, copper,stainless steel, aluminum, nickel, titanium, fired carbon, copper orstainless steel that is surface-treated with one of carbon, nickel,titanium, silver, or the like, an aluminum-cadmium alloy, or the likemay be used. Also, similar to the positive electrode collector, thenegative electrode collector may have fine surface roughness to improvebonding strength with the negative electrode active material, and thenegative electrode collector may be used in various shapes such as afilm, a sheet, a foil, a net, a porous body, a foam body, a non-wovenfabric body, and the like.

The negative electrode active material may include a carbon-basedmaterial capable of reversibly intercalating/deintercalating lithiumions or a silicon-based compound which may be doped and undoped withlithium.

First, as the carbon-based material capable of reversiblyintercalating/deintercalating lithium ions, a carbon-based negativeelectrode active material generally used in a lithium ion secondarybattery may be used without particular limitation, and, as a typicalexample, crystalline carbon, amorphous carbon, or both thereof may beused.

Examples of the crystalline carbon may be graphite such as irregular,planar, flaky, spherical, or fibrous natural graphite or artificialgraphite. Examples of the amorphous carbon may be soft carbon(low-temperature sintered carbon) or hard carbon, mesophase pitchcarbide, and fired cokes. Specifically, natural graphite or artificialgraphite may be used as the carbon-based negative electrode activematerial.

Also, the silicon-based compound, which may be doped and undoped withlithium, may include at least one selected from silicon (Si), SiO_(x)(0<x≤2), and a Si—Y alloy (where Y is an element selected from the groupconsisting of alkali metal, alkaline earth metal, a Group 13 element, aGroup 14 element excluding Si, transition metal, a rare earth element,and a combination thereof). Furthermore, a mixture of SiO₂ and at leastone thereof may also be used. The element Y may be selected from thegroup consisting of Mg, calcium (Ca), strontium (Sr), barium (Ba),radium (Ra), scandium (Sc), yttrium (Y), Ti, zirconium (Zr), hafnium(Hf), rutherfordium (Rf), V, niobium (Nb), Ta, dubnium (db), Cr, Mo,tungsten (W), seaborgium (Sg), technetium (Tc), rhenium (Re), bohrium(Bh), Fe, Pb, ruthenium (Ru), osmium (Os), hassium (Hs), rhodium (Rh),iridium (Ir), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag),gold (Au), zinc (Zn), cadmium (Cd), boron (B), Al, gallium (Ga), tin(Sn), indium (In), germanium (Ge), phosphorus (P), arsenic (As),antimony (Sb), bismuth (Bi), sulfur (S), selenium (Se), tellurium (Te),polonium (Po), and a combination thereof. Specifically, SiO_(x) (0<x≤2)may be used as the silicon-based compound.

The binder is a component that assists in the binding between theconductive agent, the active material, and the current collector.Examples of the binder may be polyvinylidene fluoride (PVDF), polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene,polyethylene, polypropylene, an ethylene-propylene-diene monomer, asulfonated ethylene-propylene-diene monomer, a styrene-butadiene rubber,a fluoro rubber, and various copolymers thereof.

The binder may be commonly included in an amount of 1 wt % to 20 wt %,preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %based on a total weight of solids excluding the solvent in the negativeelectrode material mixture slurry.

The conductive agent is a component for further improving theconductivity of the negative electrode active material. The conductiveagent is not particularly limited as long as it has conductivity withoutcausing adverse chemical changes in the battery, and, for example, aconductive material, such as: graphite such as natural graphite orartificial graphite; carbon black such as acetylene black, Ketjen black,channel black, furnace black, lamp black, and thermal black; conductivefibers such as carbon fibers or metal fibers; metal powder such asfluorocarbon powder, aluminum powder, and nickel powder; conductivewhiskers such as zinc oxide whiskers and potassium titanate whiskers;conductive metal oxide such as titanium oxide; or polyphenylenederivatives, may be used.

The conductive agent may be included in an amount of 1 wt % to 20 wt %,preferably 1 wt % to 15 wt %, and more preferably 1 wt % to 10 wt %based on the total weight of solids excluding the solvent in thenegative electrode material mixture slurry.

The solvent may include water or an organic solvent, such asN-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the negative electrode activematerial as well as optionally the binder and the conductive agent areincluded. For example, the solvent may be included in an amount suchthat a concentration of a solid content including the negative electrodeactive material as well as optionally the binder and the conductiveagent is in a range of 50 wt % to 95 wt %, for example, 70 wt % to 90 wt%.

Also, a graphite electrode formed of carbon (C) may be used as thenegative electrode, or a metal itself may be used as the negativeelectrode.

In a case in which the metal itself is used as the negative electrode,the negative electrode may be prepared from a metal thin film itself orby bonding, rolling, or depositing a metal on the negative electrodecollector by an electrical deposition method or chemical vapordeposition. The metal thin film itself or the metalbonded/rolled/deposited on the negative electrode collector may includeone metal selected from the group consisting of lithium (Li), nickel(Ni), tin (Sn), copper (Cu), and indium (In) or an alloy of two metalsthereof.

(3) Separator

Also, a typical porous polymer film used as a typical separator, forexample, a porous polymer film prepared from a polyolefin-based polymer,such as an ethylene homopolymer, a propylene homopolymer, anethylene-butene copolymer, an ethylene-hexene copolymer, and anethylene-methacrylate copolymer, may be used alone or in a laminationtherewith as the separator. Also, a typical porous nonwoven fabric, forexample, a nonwoven fabric formed of high melting point glass fibers orpolyethylene terephthalate fibers may be used, but the present inventionis not limited thereto.

A shape of the lithium secondary battery of the present invention is notparticularly limited, but a cylindrical type using a can, a prismatictype, a pouch type, or a coin type may be used.

According to another embodiment of the present invention, a batterymodule including the lithium secondary battery as a unit cell and abattery pack including the battery module are provided. Since thebattery module or the battery pack includes the lithium secondarybattery having high capacity, high rate capability, and high cyclecharacteristics, the battery module or the battery pack may be used as apower source of a medium and large sized device selected from the groupconsisting of an electric vehicle, a hybrid electric vehicle, a plug-inhybrid electric vehicle, and a power storage system.

Hereinafter, the present invention will be described in more detail,according to specific examples. However, the following examples aremerely presented to exemplify the present invention, and the scope ofthe present invention is not limited thereto. It will be apparent tothose skilled in the art that various modifications and alterations arepossible within the scope and technical spirit of the present invention.Such modifications and alterations fall within the scope of claimsincluded herein.

EXAMPLES

[Preparation of Copolymer for Polymer Electrolyte]

Example 1. Preparation of Copolymer for Polymer Electrolyte (A1)

(Step 1) 10 g of P(VDF-co-CTFE) (weight-average molecular weight (Mw)5,000), 114 g of poly(ethylene glycol) methyl ether methacrylate (mPEGA)represented by the following Formula 4, and 500 ml of dimethylformamide(DMF), as a solvent, were added to a 1,000 ml flask and stirred for 1hour under nitrogen conditions.

(in Formula 4, m=9)

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof butylated hydroxytoluene (BHT), as a polymerization inhibitor, wasadded, and a reaction was performed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a PVDF-co-P(CTFE-g-P(EGMA)) polymer(A1, weight ratio of o1:q1′:q2′=40:55:5, weight-average molecular weight(Mw) 160,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersityindex)=4.2).

Example 2. Preparation of Copolymer for Polymer Electrolyte (A2)

(Step 1) 10 g of P(VDF-co-CTFE-co-TrFE) having a weight-averagemolecular weight (Mw) of 7,500 as a copolymer for a polymer electrolyte,114 g of mPEGA represented by Formula 4, and 6 g of 4-hydroxybutylacrylate (HBA) were added to 500 ml of dimethylformamide (DMF), as asolvent, in a 1,000 ml flask and stirred for 1 hour under nitrogenconditions.

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.05 g ofazobisisobutyronitrile (AIBN), as a reducing agent, were added to theflask, an ATRP polymerization reaction was performed while stirring at60° C. for 48 hours under nitrogen conditions. In this case, a monomerconversion rate was 68%.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain aPVDF-co-P(CTFE-g-P(EGMA-r-HBA))-co-PTrFE polymer (A2, weight ratio ofo1′:q1″:p1′:q2″=30:55:10:5, weight-average molecular weight (Mw) 24,000,Mw/Mn (PDI: Poly Dispersity Index: polydispersity index)=5.5).

Example 3. Preparation of Copolymer for Polymer Electrolyte (A3)

(Step 1) 10 g of P(VDF-co-CTFE-co-TrFE) having a weight-averagemolecular weight (Mw) of 7,500 as a copolymer for a polymer electrolyte,60 g of mPEGA represented by Formula 4, 54 g of butyl acrylate (BA), and6 g of HBA were added to 500 ml of dimethylformamide (DMF), as asolvent, in a 1,000 ml flask and stirred for 1 hour under nitrogenconditions.

(Step 2) Subsequently, after 0.006 g of CuCl₂ as an ATRP reactioncatalyst, 0.026 g of TPMA as a ligand, and 0.11 g ofazobisisobutyronitrile (AIBN), as a reducing agent, were added to theflask, an ATRP polymerization reaction was performed while stirring at60° C. for 48 hours under nitrogen conditions. In this case, a monomerconversion rate was 65%.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain aPVDF-co-P(CTFE-g-P(EGMA-r-BA-r-HBA))-co-PTrFE polymer (A3, weight ratioof o1′:q1″:p1′:q2″=30:55:10:5, weight-average molecular weight (Mw)26,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersity index)=5.5).

Example 4. Preparation of Copolymer for Polymer Electrolyte (A4)

(Step 1) 10 g of PCTFE (weight-average molecular weight (Mw) 5,000), 114g of mPEGA represented by Formula 4, and 500 ml of dimethylformamide(DMF), as a solvent, were added to a 1,000 ml flask and stirred for 1hour under nitrogen conditions.

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof butylated hydroxytoluene (BHT), as a polymerization inhibitor, wasadded, and a reaction was performed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a P(CTFE-g-P(EGMA)) polymer (A4,weight ratio of q1′:q2′=40:60, weight-average molecular weight (Mw)160,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersity index)=4.2).

Example 5. Preparation of Copolymer for Polymer Electrolyte (A5)

(Step 1) 10 g of P(CTFE-co-TrFE) (weight-average molecular weight (Mw)5,000), 114 g of mPEGA represented by Formula 4, and 500 ml ofdimethylformamide (DMF), as a solvent, were added to a 1,000 ml flaskand stirred for 1 hour under nitrogen conditions.

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof butylated hydroxytoluene (BHT) was added, and a reaction wasperformed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a P(CTFE-g-P(EGMA))-co-PTrFE polymer(A5, weight ratio of p1:q1′:q2′=30:60:10, weight-average molecularweight (Mw) 160,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersityindex)=4.2).

Example 6. Preparation of Copolymer for Polymer Electrolyte (A6)

(Step 1) 10 g of P(VDF-co-CTFE) (weight-average molecular weight (Mw)5,000), 114 g of poly(ethylene glycol) trifluoroethoxyethyl methacrylate(tri-FPEGMA) represented by the following Formula 5, and 500 ml ofdimethylformamide (DMF), as a solvent, were added to a 1,000 ml flaskand stirred for 1 hour under nitrogen conditions.

(in Formula 5, m=9)

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof BHT was added, and a reaction was performed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a PVDF-co-P(CTFE-g-P(tri-FEGMA))polymer ((A6), weight ratio of o1:q1′:q2′=20:75:5, weight-averagemolecular weight (Mw) 120,000, Mw/Mn (PDI: Poly Dispersity Index:polydispersity index)=4.9).

Example 7. Preparation of Copolymer for Polymer Electrolyte (A7)

(Step 1) 10 g of P(VDF-co-CTFE) (weight-average molecular weight (Mw)5,000), 114 g of poly(ethylene glycol) propoxyethyl methacrylate(PPEGMA) represented by the following Formula 6, and 500 ml ofdimethylformamide (DMF), as a solvent, were added to a 1,000 ml flaskand stirred for 1 hour under nitrogen conditions.

(in Formula 6, m=9)

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof butylated hydroxytoluene (BHT) was added, and a reaction wasperformed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a PVDF-co-P(CTFE-g-P(PEGMA)) polymer((A7), weight ratio of o1:q1′:q2′=20:75:5, weight-average molecularweight (Mw) 120,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersityindex)=5.8).

Example 8. Preparation of Copolymer for Polymer Electrolyte (A8)

(Step 1) 10 g of P(VDF-co-CTFE) (weight-average molecular weight (Mw)5,000), 114 g of poly(ethylene glycol) propenoxyethyl methacrylate(PEPEGMA) represented by the following Formula 7, and 500 ml ofdimethylformamide (DMF), as a solvent, were added to a 1,000 ml flaskand stirred for 1 hour under nitrogen conditions.

(in Formula 7, m=9)

(Step 2) Subsequently, after 0.0025 g of CuCl₂ as an ATRP reactioncatalyst, 0.011 g of TPMA as a ligand, and 0.20 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as a reducing agent, were added to the flask, an ATRPpolymerization reaction was performed while stirring at 60° C. for 48hours under nitrogen conditions. In this case, a monomer conversion ratewas 65%. The polymerization reaction was terminated by air bubblingwhile the temperature of the reactant was maintained at 60° C., 0.003 gof butylated hydroxytoluene (BHT) was added, and a reaction wasperformed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to obtain a PVDF-co-P(CTFE-g-P(PEEGMA)) polymer((A8), weight ratio of o1:q1′:q2′=20:75:5, weight-average molecularweight (Mw) 120,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersityindex)=6.0).

Example 9

(Step 1) 10 g of P(VDF-co-CTFE) having a weight-average molecular weight(hereinafter, referred to as “Mw”) of 600,000 as a fluorine-basedpolymer, 54 g of mPEGA represented by Formula 4, and 1.5 g of hydroxyethyl methacrylate (HEMA) were added to 300 ml of acetone, as a solvent,in a 1,000 ml flask and stirred for 1 hour under nitrogen conditions.

(Step 2) Subsequently, after 0.002 g of CuCl₂ as an ATRP reactioncatalyst, 0.0051 g of TPMA as a ligand, and 0.231 g of Sn(EH)2 (Tin(II)2-ethylhexanoate), as an initiator, were added to the flask, an ATRPpolymerization reaction was performed while stirring for 30 hours undernitrogen conditions.

(Step 3) After the reaction was completed, a polymer thus formed wasimmersed in an ether solvent three times to remove monomers which didnot participate in the reaction. The finally obtained polymer was driedunder vacuum conditions for 1 week to obtain aPVDF-co-(PCTFE-g-(PEGMA-co-HEMA) polymer in the form of a gel (weightratio of o1′:q1″:q2″=30:65:5, weight-average molecular weight (Mw)180,000, Mw/Mn (PDI: Poly Dispersity Index: polydispersity index)=6.7).

Comparative Example 1. Linear Copolymer Preparation (B1)

(Step 1) 23.5 g of BA, 1.25 g of HBA, and 25 mL of DMF, as a solvent,were added to a 100 ml flask and stirred for 1 hour under nitrogenconditions.

(Step 2) 0.25 g of AIBN, as a radical initiator, was added and stirredat 60° C. for 20 hours under nitrogen conditions to perform apolymerization reaction. The polymerization reaction was terminated byair bubbling while the temperature of the reactant was maintained at 60°C., and 0.0015 g of butylated hydroxytoluene, as a polymerizationinhibitor, was added. Subsequently, 0.13 g of dibutyltin dilaurate(DBTDL) and 2.94 g of 2-acryloyloxyethyl isocyanate (AOI) were added,and a condensation reaction was performed for 24 hours.

(Step 3) After the reaction was completed, a polymer thus formed waspassed through a short alumina column, added to diethyl ether, and thenre-precipitated at −30° C. or less to remove unreacted monomers, DMF,and other impurities. The obtained polymer was dried under vacuumconditions for 24 hours to prepare a BA-HBA-AOI copolymer withoutcontaining an ion conductive functional group (B1, weight-averagemolecular weight (Mw) 35,000, Mw/Mn (PDI: Poly Dispersity Index:polydispersity index)=2.3).

TABLE 1 Main chain (A) (fluorine- Grafted unit (B) Weight ratio basedpolymer) Type Weight ratio of A:B Example 1 P(VDF-co-CTFE) mPEGA — 15:85Example 2 P(VDF-co-CTFE-co-TrFE) mPEGA/HBA 95:5 15:85 Example 3P(VDF-co-CTFE-co-TrFE) mPEGA/BA/HBA 90:5:5 11:89 Example 4 PCTFE mPEGA —11:89 Example 5 P(CTFE-co-TrFE) mPEGA Example 6 P(VDF-co-CTFE)tri-FPEGMA — 15:85 Example 7 P(VDF-co-CTFE) PPEGMA — 15:85 Example 8P(VDF-co-CTFE) PEPEGMA — 15:85 Example 9 P(VDF-co-CTFE) mPEGA/HEMA 95:560:40 Example 5 P(CTFE-co-TrFE) mPEGA — 15:85 Comparative — BA-HBA-AOI50:44:6  0:100 Example 1

In Table 1, the abbreviations of the compounds have the followingmeanings, respectively.

P(VDF-co-CTFE): polyvinylidene fluoride-co-polychlorotrifluoroethylene

mPEGA: poly(ethylene glycol) methyl ether methacrylate

HBA: hydroxybutyl acrylate

BA: butyl acrylate

tri-FPEGMA: poly(ethylene glycol) trifluoroethoxyethyl methacrylate

PPEGMA: poly(ethylene glycol) propoxyethyl methacrylate

PEPEGMA: poly(ethylene glycol) propenoxyethyl methacrylate

HEMA: hydroxy ethyl methacrylate

[Secondary Battery Preparation]

Example 10

(Composition for Polymer Electrolyte)

A composition for a polymer electrolyte was prepared by adding 1 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 3 g of vinylene carbonate, as an additive, to 95.5 g of anorganic solvent (ethylene carbonate (EC):ethyl methyl carbonate(EMC)=3:7 volume ratio) in which 1 M LiPF₆ was dissolved.

(Lithium Secondary Battery Preparation)

97.5 wt % of a positive electrode active material(LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂; NCM), 1.5 wt % of carbon black as aconductive agent, and 1 wt % of PVDF, as a binder, were added toN-methyl-2-pyrrolidone (NMP), as a solvent, to prepare a positiveelectrode material mixture slurry. An about 20 μm thick Al thin film, asa positive electrode collector, was coated with the positive electrodematerial mixture slurry, dried, and then roll-pressed to prepare apositive electrode.

An artificial graphite electrode was used as a negative electrode.

After an electrode assembly was prepared by inserting a separator formedof polyethylene (PE) between the positive electrode and the negativeelectrode, the electrode assembly was accommodated in a battery caseand, after the above-prepared composition for a gel polymer electrolytewas injected, the electrode assembly was left standing for 2 days andthen heated at 60° C. for 24 hours to prepare a lithium secondarybattery including a gel polymer electrolyte.

Example 11

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A2) of Example 2, instead of the copolymer ofExample 1, was used.

Example 12

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A3) of Example 3, instead of the copolymer ofExample 1, was used.

Example 13

A composition for a polymer electrolyte was prepared by adding 2 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 2 g of vinylene carbonate, as an additive, to 95.5 g of anorganic solvent (EC:EMC=3:7 volume ratio) in which 1 M LiPF₆ wasdissolved.

A lithium secondary battery was prepared in the same manner as inExample 10 except that the composition for a polymer electrolyte wasincluded.

Example 14

A composition for a polymer electrolyte was prepared by adding 3 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 2 g of vinylene carbonate, as an additive, to 94.5 g of anorganic solvent (EC:EMC=3:7 volume ratio) in which 1 M LiPF₆ wasdissolved.

A lithium secondary battery was prepared in the same manner as inExample 10 except that the composition for a polymer electrolyte wasincluded.

Example 15

A composition for a polymer electrolyte was prepared by adding 0.01 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 2 g of vinylene carbonate, as an additive, to 97.49 g of anorganic solvent (EC:EMC=3:7 volume ratio) in which 1 M LiPF₆ wasdissolved.

A lithium secondary battery was prepared in the same manner as inExample 10 except that the composition for a polymer electrolyte wasincluded.

Example 16

A composition for a polymer electrolyte was prepared by adding 10 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 2 g of vinylene carbonate, as an additive, to 87.5 g of anorganic solvent (EC:EMC=3:7 volume ratio) in which 1 M LiPF₆ wasdissolved.

A lithium secondary battery was prepared in the same manner as inExample 10 except that the composition for a polymer electrolyte wasincluded.

Example 17

A composition for a polymer electrolyte was prepared by adding 20 g ofthe copolymer (A1) of Example 1, 0.5 g of a polymerization initiator(AIBN), and 2 g of vinylene carbonate, as an additive, to 77.5 g of anorganic solvent (EC:EMC=3:7 volume ratio) in which 1 M LiPF₆ wasdissolved.

A lithium secondary battery was prepared in the same manner as inExample 10 except that the composition for a polymer electrolyte wasincluded.

Example 18

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A4) of Example 4, instead of the copolymer ofExample 1, was used.

Example 19

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A5) of Example 5, instead of the copolymer ofExample 1, was used.

Example 20

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A6) of Example 6, instead of the copolymer ofExample 1, was used.

Example 21

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A7) of Example 7, instead of the copolymer ofExample 1, was used.

Example 22

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (A8) of Example 8, instead of the copolymer ofExample 1, was used.

Example 23

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer of Example 9, instead of the copolymer of Example 1,was used.

Comparative Example 2

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat the copolymer (B1) prepared in Comparative Example 1, instead ofthe copolymer of Example 1, was used.

Comparative Example 3

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 10 exceptthat a P(VDF-co-CTFE-co-TrFE) copolymer, instead of the copolymer ofExample 1, was used.

Comparative Example 4

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 13 exceptthat the copolymer (B1) prepared in Comparative Example 1, instead ofthe copolymer of Example 1, was used.

Comparative Example 5

A composition for a polymer electrolyte and a lithium secondary batteryincluding a gel polymer electrolyte prepared by polymerization of thecomposition were prepared in the same manner as in Example 1 except thata P(VDF-co-CTFE) copolymer, instead of the copolymer of Example 1, wasused.

Comparative Example 6

5.0 g of polyvinylidene fluoride (PVDF, Mn: 107,000 g/mol, Mw: 275,000g/mol, Aldrich, USA) was dissolved in 50 ml of N-methyl-2-pyrrolidone(NMP) in a reactor, and stirred at 90° C. for 6 hours. Then, 5.0 g ofpoly(ethylene glycol) methyl ether methacrylate having a weight-averagemolecular weight of 1,300 (PEGMA, Mn 475 g/mol, Aldrich) was added tothe polymer solution in the reactor and stirred together to prepare ahomogenous solution.

Then, 0.04 g of copper chloride and 0.22 ml ofhexamethyltriethylenetetraamine (Aldrich) were added to the mixedsolution. Next, after nitrogen was injected for 30 minutes whilestirring the solution, the solution was put in an oil bath preheated to120° C., and reacted for 24 hours.

After a solution, in which 5 g of a sulfopropyl methacrylate potassiumsalt (SPMA 98%, Aldrich, USA) was dissolved in 50 ml of dimethylsulfoxide (DMSO, Aldrich), was added to the polymer solution thusobtained, and mixed and stirred together to prepare a homogenoussolution. Next, after nitrogen was injected for 30 minutes whilestirring the solution, the solution was put in an oil bath preheated to120° C., and reacted for 24 hours. Next, the polymer solution after thereaction was completed was precipitated in a methanol-nucleic acid (1:1volume ratio) solvent and filtered to recover a PVDF-g-(PEGMEM-co-SPMA)polymer.

Then, the polymer was re-dissolved in dimethyl sulfoxide and purified byreprecipitation in a methanol-nucleic acid solvent three times torecover a polymer. Next, the polymer was dried in vacuum at roomtemperature for 24 hours. Next, after a solution was prepared bydissolving the prepared polymer in dimethyl sulfoxide at a concentrationof 5 wt %, the solution was poured into a glass dish. Then, the glassdish was put in a drying oven at 80° C. to remove the solvent for twodays, and the remaining solvent was then completely removed in a vacuumoven to prepare a polymer layer.

(Lithium Secondary Battery Preparation)

After a cell was prepared by stacking the prepared polymer layer betweenthe positive electrode and the negative electrode, an electrolytesolution (EC/EMC=3/7, 1 M LiPF₆, 3% VC, 0.1% AIBN) was injected. Thecell after completion of the injection was left standing for 2 days andthen heated at 60° C. for 24 hours to prepare a lithium secondarybattery including a gel polymer electrolyte.

EXPERIMENTAL EXAMPLES Experimental Example 1. Ionic ConductivityEvaluation of Composition for Polymer Electrolyte

Specimens were prepared by using the compositions for polymerelectrolytes prepared in Examples 10 to 19 and the compositions forpolymer electrolytes prepared in Comparative Examples 2 to 5. Thespecimens were collectively prepared according to ASTM standard D638(Type V specimens).

Subsequently, a circular gold (Au) electrode having a diameter of 1 mmwas coated on the specimens using a sputtering method, and ionicconductivities were measured at 25° C. by using an alternating currentimpedance method. The ionic conductivities were measured in a frequencyrange of 0.1 Hz to 100 MHz using a VMP3 measurement instrument and aprecision impedance analyzer (4294A). The measurement results arepresented in Table 2 below.

TABLE 2 Copolymer Weight-average Ionic molecular weight Amountconductivity Type (Mw) (wt %) (S/cm) Example 10 A1 160,000 1 9.0 × 10⁻³Example 11 A2 24,000 1 8.8 × 10⁻³ Example 12 A3 26,000 1 8.6 × 10⁻³Example 13 A1 160,000 2 8.8 × 10⁻³ Example 14 A1 160,000 3 8.0 × 10⁻³Example 15 A1 160,000 0.01 9.3 × 10⁻³ Example 16 A1 160,000 10 7.4 ×10⁻³ Example 17 A1 160,000 20 6.5 × 10⁻³ Example 18 A4 160,000 1 7.9 ×10⁻³ Example 19 A5 160,000 1 7.6 × 10⁻³ Comparative B1 35,000 1 6.2 ×10⁻³ Example 2 Comparative P(VDF-co- — 1 3.7 × 10⁻³ Example 3 CTFE-co-TrFE) Comparative B1 35,000 2 5.1 × 10⁻³ Example 4 Comparative P(VDF-co-— 1 4.3 × 10⁻³ Example 5 CTFE)

Referring to Table 2, the specimens prepared by using the compositionsfor polymer electrolytes of Examples 10 to 16, 18, and 19 had an ionicconductivity at 25° C. of 7.4×10⁻³ or more. In contrast, with respect tothe specimens prepared by using the compositions for polymerelectrolytes prepared in Comparative Examples 2 to 5, ionicconductivities at 25° C. were 6.2×10⁻³ or less, wherein it may beunderstood that the ionic conductivities were reduced in comparison tothose of the compositions for polymer electrolytes prepared in Examples10 to 16, 18, and 19.

With respect to the composition for a polymer electrolyte of Example 17which included 20 wt % of the copolymer for a polymer electrolyte havinga weight-average molecular weight of 160,000, since viscosity wasincreased, it may be understood that the ionic conductivity was reducedin comparison to those of the compositions for polymer electrolytesprepared in Examples 10 to 16, 18, and 19.

Experimental Example 2. Electrochemical Stability Evaluation

Electrochemical (oxidation) stability of each of the secondary batteriesprepared in Examples 10 to 19 and the secondary batteries prepared inComparative Examples 2 to 5 was measured using linear sweep voltammetry(LSV). The measurement was made by using a potentiostat (EG&G, model270A) under a three-electrode system (working electrode: platinum disk,counter electrode: platinum, reference electrode: lithium metal), andmeasurement temperature was 60° C. The results thereof are presented inTable 3 below.

Experimental Example 3. Electrode Stability Evaluation

Each of the secondary batteries prepared in Examples 10 to 19 and 20 to22 and the secondary batteries prepared in Comparative Examples 2, 4,and 5 was fully charged to a state of charge (SOC) of 100% (44.3 mAh)under a voltage condition of 4.25 V. Thereafter, at 25° C., thetemperature was increased to 120° C. at a rate of 0.7° C./min and thenmaintained in a temperature range of 120° C. for about 100 minutes(1^(st) temperature maintenance section). Thereafter, the temperaturewas again increased to 150° C. at a rate of 0.7° C./min and thenmaintained in a temperature range of 150° C. for about 100 minutes(2^(nd) temperature maintenance section). Thereafter, the temperaturewas again increased to 200° C. at a rate of 0.7° C./min and thenmaintained in a temperature range of 200° C. for about 100 minutes(3^(rd) temperature maintenance section).

An internal calorific value of each lithium secondary battery exposed tothe above-described temperature conditions was measured using a MMCinstrument (Multiple Module Calorimeter, MMC 274, NETZSCH), and theresults thereof are presented in Table 3 below.

TABLE 3 Copolymer Battery Amount OFF-Set calorific Type (wt %) voltage(V) value (J) Example 10 A1 1 5.06 94 Example 11 A2 1 5.07 62 Example 12A3 1 5.04 83 Example 13 A1 2 5.11 86 Example 14 A1 3 5.17 74 Example 15A1 0.01 4.97 99 Example 16 A1 10 5.24 52 Example 17 A1 20 5.51 49Example 18 A4 30 5.02 91 Example 19 A5 1 5.07 80 Example 20 A6 1 — 57Example 21 A7 1 — 89 Example 22 A8 1 — 72 Example 23 A9 1 — 90Comparative B1 1 4.62 140 Example 2 Comparative P(VDF-co-CTFE- 1 4.95 —Example 3 co-TrFE) Comparative B1 2 4.63 135 Example 4 ComparativeP(VDF-co-CTFE) 1 4.89 101 Example 5

Referring to Table 3, since the secondary batteries prepared inComparative Examples 2 to 5 had an oxidation initiation voltage of about4.95 V or less, but the secondary batteries prepared in Examples 10 to19 of the present invention had an oxidation initiation voltage of about4.97 V or more, it was confirmed that the secondary batteries preparedin Examples 10 to 19 of the present invention exhibited excellentelectrochemical (oxidation) stabilities.

Also, referring to Table 3, it may be understood that calorific valuesof the secondary batteries prepared in Examples 10 to 19 and 20 to 22 ofthe present invention were about 99 J/g or less, but calorific values ofthe secondary batteries prepared in Comparative Examples 2, 4, and 5were increased to about 101 J/g or more.

With respect to the secondary battery of Example 17, it may beunderstood that the ionic conductivity was reduced due to the largeamount of the copolymer for a polymer electrolyte, but theelectrochemical stability and electrode stability were relativelyimproved.

Experimental Example 4. Initial Capacity Evaluation

After each of the secondary batteries prepared in Examples 10 and 20 to23 and the secondary battery prepared in Comparative Example 6 wasactivated at a CC of 0.1 C, degassing was performed.

Then, each secondary battery was charged at a CC of 0.33 C to 4.20 Vunder a constant current-constant voltage (CC-CV) charging condition at25° C., then subjected to 0.05 C current cut-off, and discharged at 0.33C under a CC condition to 2.5 V. The above charging and discharging weredefined as one cycle and three cycles were performed.

Then, initial discharge capacity was measured using PNE-0506charge/discharge equipment, and the results thereof are presented inTable 4 below.

TABLE 4 25° C. initial discharge capacity (%) Example 10 99.5 Example 2099.8 Example 21 99.6 Example 22 99.5 Example 23 83.8 Comparative Example6 74.6

Referring to Table 4, initial discharge capacity of the secondarybattery prepared in Comparative Example 6 was 74.6%, wherein it may beunderstood that the initial discharge capacity was significantly reducedin comparison to those of the secondary batteries prepared in Examples10 and 20 to 23. Thus, it may be understood that the lithium secondarybattery prepared in Comparative Example 6 had low initial capacity.

1. A copolymer for a polymer electrolyte, comprising a compoundrepresented by Formula 1:

wherein, in Formula 1, R_(a1) to R_(f1) are each independently hydrogen,a fluorine element, or an alkyl group having 1 to 10 carbon atoms, R₁ ishydrogen or a substituted or unsubstituted alkyl group having 1 to 4carbon atoms, R₂ is hydrogen or an alkyl group having 1 to 5 carbonatoms, Y is an alkyl group having 1 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element; analkenyl group having 2 to 15 carbon atoms which is substituted orunsubstituted with at least one halogen element; a cyclic alkyl grouphaving 3 to 10 carbon atoms; a heterocyclic group having 3 to 10 carbonatoms; a cyclic ether group having 3 to 10 carbon atoms; a heterocyclicether group having 3 to 10 carbon atoms; —(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH,wherein R₁₈ and R₁₉ each are hydrogen or an alkyl group having 1 to 5carbon atoms, and d and e are each independently an integer of 0 to 10,but are not 0 at a same time; —(CR₇R₈)_(g)—O—(CR₉R₁₀)_(h)—CH₃, whereinR₇ to R₁₀ each are a substituted or unsubstituted alkylene group having1 to 15 carbon atoms, and g and h each are an integer of 1 to 10; anaryl group having 6 to 12 carbon atoms; —(CH₂)_(f)—CN, wherein f is aninteger of 0 to 10; —(CH₂)_(i)—O—CH₂═CH₂, wherein i is an integer of 0to 10; —(CH₂)_(j)—Si(R₂₀)_(k)(OCH₂CH₃)_(3-k), wherein R₂₀ is hydrogen(H), j is an integer of 1 to 10, and k is an integer of 1 or 2;—(CH₂)_(w)—NCO, wherein w is an integer of 1 to 10; —CH₂CH₂—N(CH₃)₂;—(CH₂)_(x)-A, wherein A is ═—OC(═O)(CH₂)_(y)COOH (y is an integer of 1to 10) or A is C(═O)(CH₂)_(z)COOH (z is an integer of 1 to 10);

m is an integer of 1 to 2,300, n is an integer of 1 to 2,000, o1 is aninteger of 0 to 400, p1 is an integer of 0 to 400, q1′ is an integer of1 to 300, and q2′ is an integer of 0 to 300, with the proviso that o1and q2′ are not 0 at a same time.
 2. The copolymer of claim 1, wherein,in Formula 1, Y is an alkyl group having 1 to 10 carbon atoms which issubstituted with at least one halogen element; an alkyl group having 3to 10 carbon atoms which is unsubstituted with an halogen element; analkenyl group having 2 to 15 carbon atoms which is substituted orunsubstituted with at least one halogen element;—(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH, wherein R₁₈ and R₁₉ each are hydrogen or analkyl group having 1 to 5 carbon atoms, and d and e are eachindependently an integer of 1 to 10; —(R₇)_(g)—O—(R₈)_(h)—CH₃, whereinR₇ and R₈ each are a substituted or unsubstituted alkylene group having1 to 15 carbon atoms, and g and h each are an integer of 1 to 10;—(CH₂)_(i)—O—CH₂═CH₂, wherein i is an integer of 1 to 10 or i is—(CH₂)_(w)—NCO, wherein w is an integer of 1 to
 10. 3. The copolymer ofclaim 1, wherein the copolymer comprises a compound represented byFormula 2:

wherein, in Formula 2, R_(a1)′ to R_(f1)′ are each independentlyhydrogen or a fluorine element, R₁′ is hydrogen or a substituted orunsubstituted alkyl group having 1 to 3 carbon atoms, R_(4a) to R_(4c)are each independently hydrogen or a substituted or unsubstituted alkylgroup having 1 to 3 carbon atoms, R₂′ is hydrogen or an alkyl grouphaving 1 to 5 carbon atoms, Y′ and R_(5a) to R_(5c) each are an alkylgroup having 1 to 15 carbon atoms which is substituted or unsubstitutedwith at least one halogen element; an alkenyl group having 2 to 15carbon atoms which is substituted or unsubstituted with at least onehalogen element; a cyclic alkyl group having 3 to 10 carbon atoms; aheterocyclic group having 3 to 10 carbon atoms; a cyclic ether grouphaving 3 to 10 carbon atoms; a heterocyclic ether group having 3 to 10carbon atoms; —(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH, wherein R₁₈ and R₁₉ each arehydrogen or an alkyl group having 1 to 5 carbon atoms, and d and e areeach independently an integer of 0 to 10; —(R₇)_(g)—O—(R₈)_(h)—CH₃,wherein R₇ and R₈ each are a substituted or unsubstituted alkylene grouphaving 1 to 15 carbon atoms, and g and h each are an integer of 1 to 10;an aryl group having 6 to 12 carbon atoms; —(CH₂)_(f)—CN, wherein f isan integer of 0 to 10; —(CH₂)_(i)—O—CH₂═CH₂, wherein i is an integer of0 to 10; —(CH₂)_(j)—Si(R₉)_(k)(OCH₂CH₃)_(3-k), wherein R₉ is H, j is aninteger of 1 to 10, and k is an integer of 1 to 3; —(CH₂)_(w)—NCO,wherein w is an integer of 1 to 10; —CH₂CH₂—N(CH₃)₂; —(CH₂)_(x)-A,wherein A= is —OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 10) or A isC(═O)(CH₂)_(z)COOH (z is an integer of 1 to 10);

m′ is an integer of 1 to 2,300, o1′ is an integer of 0 to 400, p1′ is aninteger of 0 to 400, q1″ is an integer of 1 to 300, q2″ is an integer of0 to 300, with the proviso that o1′ and q2″ are not 0 at a same time, n1is an integer of 1 to 2,000, n2 is an integer of 0 to 2,000, n3 is aninteger of 0 to 2,000, and n4 is an integer of 0 to 2,000.
 4. Thecopolymer of claim 3, wherein, in Formula 2, Y′ is an alkyl group having1 to 10 carbon atoms which is substituted with at least one halogenelement; an alkyl group having 3 to 10 carbon atoms which isunsubstituted with an halogen element; an alkenyl group having 2 to 15carbon atoms which is substituted or unsubstituted with at least onehalogen element; —(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH (R₁₈ and R₁₉ each arehydrogen or an alkyl group having 1 to 5 carbon atoms, and d and e areeach independently an integer of 0 to 10); —(R₇)_(g)—O—(R₈)_(h)—CH₃,wherein R₇ and R₈ each are a substituted or unsubstituted alkylene grouphaving 1 to 15 carbon atoms, and g and h each are an integer of 1 to 10;—(CH₂)_(i)—O—CH₂═CH wherein i is an integer of 1 to 10; or—(CH₂)_(w)—NCO, wherein w is an integer of 1 to 10, and R_(5a) to R_(5c)are each independently an alkyl group having 1 to 10 carbon atoms whichis substituted or unsubstituted with at least one halogen element; analkenyl group having 2 to 12 carbon atoms which is substituted orunsubstituted with at least one halogen element; a cyclic alkyl grouphaving 3 to 8 carbon atoms; a heterocyclic group having 3 to 8 carbonatoms; a cyclic ether group having 3 to 8 carbon atoms; a heterocyclicether group having 3 to 8 carbon atoms; —(R₇)_(g)—O—(R₈)_(h)—CH₃,wherein R₇ and R₈ each are a substituted or unsubstituted alkylene grouphaving 1 to 10 carbon atoms, and g and h each are an integer of 1 to 10;an aryl group having 6 to 12 carbon atoms; —(CH₂)_(f)—CN, wherein f isan integer of 0 to 10; —(CH₂)_(i)—O—CH₂═CH₂, wherein i is an integer of0 to 10; —(CH₂)_(j)—Si(R₉)_(k)(OCH₂CH₃)_(3-k), wherein R₉ is H, j is aninteger of 1 to 10, and k is an integer of 1 to 3; —(CH₂)_(w)—NCO,wherein w is an integer of 2 to 10; —CH₂CH₂—N(CH₃)₂; —(CH₂)_(x)-A,wherein A= is —OC(═O)(CH₂)_(y)COOH (y is an integer of 1 to 8) or A isC(═O)(CH₂)_(z)COOH (z is an integer of 1 to 8)), m′ is an integer of 1to 2,000, o1′ is an integer of 0 to 350, p1′ is an integer of 0 to 350,q2″ is an integer of I to 250, q2″ is an integer of 0 to 250, with theproviso that o1′ and q2″ are not 0 at a same time, n1 is an integer of 1to 1,500, n2 is an integer of 0 to 1,500, n3 is an integer of 0 to1,500, and n4 is an integer of 0 to 1,500.
 5. The copolymer of claim 3,wherein, in Formula 2, Y′ is an alkyl group having 1 to 10 carbon atomswhich is substituted with at least one halogen element; an alkyl grouphaving 3 to 10 carbon atoms which is unsubstituted with an halogenelement; an alkenyl group having 2 to 15 carbon atoms which issubstituted or unsubstituted with at least one halogen element; or—(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH, wherein R₁₈ and R₁₉ each are hydrogen or analkyl group having 1 to 5 carbon atoms, and d and e are eachindependently an integer of 0 to 10, R_(5a) is an alkyl group having 1to 10 carbon atoms which is substituted or unsubstituted with at leastone halogen element; an alkenyl group having 2 to 12 carbon atoms whichis substituted or unsubstituted with at least one halogen element; acyclic alkyl group having 3 to 8 carbon atoms; a heterocyclic grouphaving 3 to 8 carbon atoms; a cyclic ether group having 3 to 8 carbonatoms; —(CH₂)_(d)—(CR₁₈R₁₉)_(e)—OH, wherein R₁₈ and R₁₉ each arehydrogen or an alkyl group having 1 to 3 carbon atoms, and d and e areeach independently an integer of 1 to 10; —(CH₂)_(i)—O—CH₂═CH₂, whereini is an integer of 1 to 9; —(R₇)_(g)—O—(R₈)_(h)—CH₃, wherein R₇ and R₈each are a substituted or unsubstituted alkylene group having 1 to 10carbon atoms, and g and h each are an integer of 1 to 8; or—(CH₂)_(x)-A, wherein A= is —OC(═O)(CH₂)_(y)COOH (y is an integer of 1to 8) or A is C(═O)(CH₂)_(z)COOH (z is an integer of 1 to 8)), R_(5b) isan alkyl group having 1 to 10 carbon atoms which is substituted orunsubstituted with at least one halogen element; an alkenyl group having2 to 12 carbon atoms which is substituted or unsubstituted with at leastone halogen element; a heterocyclic group having 3 to 8 carbon atoms, acyclic ether group having 3 to 8 carbon atoms; a heterocyclic ethergroup having 3 to 8 carbon atoms; —(CH₂)_(f)—CN, wherein f is an integerof 1 to 8; —(CH₂)_(w)—NCO, wherein w is an integer of 2 to 10;—CH₂CH₂—N(CH₃)₂; or —(CH₂)_(x)-A, wherein A is —OC(═O)(CH₂)_(y)COOH (yis an integer of 1 to 8) or A is C(═O)(CH₂)_(z)COOH (z is an integer of1 to 8), and R_(5c) is a cyclic alkyl group having 3 to 8 carbon atoms;a heterocyclic group having 3 to 8 carbon atoms; a cyclic ether grouphaving 3 to 8 carbon atoms; a heterocyclic ether group having 3 to 8carbon atoms; an aryl group having 6 to 12 carbon atoms; —(CH₂)_(f)—CN,wherein f is an integer of 1 to 8; —(CH₂)_(w)—NCO, wherein w is aninteger of 2 to 10; or —CH₂CH₂—N(CH₃)₂.
 6. The copolymer of claim 1,further comprising at least one selected from the group consisting of:polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate) (PVDF-co-P(CTFE-g-P(EGMA)));polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate-r-hydroxybutylacrylate)-co-polytrifluoroethylene(PVDF-co-P(CTFE-g-P(EGMA-r-HBA))-co-PTrFE); polyvinylidenefluoride-co-polychlorotrifluoroethylene-g-poly((ethylene glycol) methylether methacrylate-r-butyl acrylate-r-hydroxybutylacrylate)-co-polytrifluoroethylene(PVDF-co-P(CTFE-g-P(EGMA-r-BA-r-HBA))-co-PTrFE);polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate) (P(CTFE-g-P(EGMA));polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate)-co-polytrifluoroethylene (P(CTFE-g-P(EGMA))-co-PTrFE);polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) trifluoroethoxyethyl methacrylate)(PVDF-co-P(CTFE-g-P(tri-FEGMA)); polyvinylidenefluoride-co-polychlorotrifluoroethylene-g-poly((ethylene glycol)propoxyethyl methacrylate) (PVDF-co-P(CTFE-g-P(PEGMA)); polyvinylidenefluoride-co-polychlorotrifluoroethylene-g-poly((ethylene glycol)propenoxyethyl methacrylate) (PVDF-co-P(CTFE-g-P(PEEGMA));polyvinylidene fluoride-co-polychlorotrifluoroethylene-g-poly((ethyleneglycol) methyl ether methacrylate-r-hydroxybutylacrylate-acryloyloxyethyl isocyanate)-co-polytrifluoroethylene;polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-hydroxybutyl acrylate);polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate);polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate-r-acryloyloxyethylisocyanate); polychlorotrifluoroethylene-g-poly((ethylene glycol) methylether methacrylate-r-hydroxybutyl acrylate)-co-polytrifluoroethylene;polychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutylacrylate)-co-polytrifluoroethylene; andpolychlorotrifluoroethylene-g-poly((ethylene glycol) methyl ethermethacrylate-r-butyl acrylate-r-hydroxybutyl acrylate-r-acryloyloxyethylisocyanate)-co-polytrifluoroethylene.
 7. A composition for a polymerelectrolyte, comprising the copolymer of claim 1, a non-aqueouselectrolyte solution, and a polymerization initiator.
 8. The compositionof claim 7, wherein an amount of the copolymer is 0.01 wt % to 30 wt %based on a total weight of the composition for a polymer electrolyte. 9.The composition of claim 7, further comprising at least one additiveselected from the group consisting of vinylene carbonate, LiBF₄,vinylethylene carbonate, 1,3-propane sultone, 1,3-propene sultone,succinonitrile, adiponitrile, fluoroethylene carbonate, ethylenesulfate, LiPO₂F₂, methyl trimethylene sulfate, lithiumdifluorooxalatoborate (LiODFB), lithium bis-(oxalato)borate),tetraphenylborate (LiBOB), tetraphenylborate,3-trimethoxysilanyl-propyl-N-aniline, tris(trimethylsilyl)phosphite,tris(2,2,2-trifluoroethyl)phosphate, and tis(trifluoroethyl)phosphite.10. A gel polymer electrolyte prepared by a thermal polymerization ofthe composition of claim
 7. 11. A lithium secondary battery comprisingthe gel polymer electrolyte of claim 10.